My first answer to this is simple: don't rely on other PCs to keep you alive in dangerous situations. Be as self-sufficient as you can.
I hate games like this. To me, D&D is defined by relying on other people. Back when I started it was impossible for one character to do everything. In 2e with only the PHB, if you were a fighter, there was no way you were healing yourself, there was no way you could apply any sort of status effect on enemies. You couldn't choose any options except which weapon you used. Your only choice was your background, personality and appearance really. If you were hurt, you had to rely on the DM to give you healing potions or the Cleric to heal you. If there were too many enemies to take on by yourself, you had to rely on the Wizard to take out the extra enemies with fireballs.
This is a large part of what I like about D&D, the fact that people have to work together. I know that playing 3e/3.5e caused a number of the people I play with to start playing the game in a very "I'm only out for myself and my character concept is that I'm going to keep multiclassing until I have all the bases covered and don't have to rely on anyone." But it was definitely reflected in the personalities of their characters. Party members would threaten to leave the group if they didn't get their way on a regular basis. After all, their character was self sufficient, it didn't matter if he/she was on their own.
I think all of us dabbled in that for a while, until we all realized the group worked better with 5 specialists than it did with 5 jack of all trades. One Heal spell healed more damage than the entire compliment of healing spells that the 2 Fighter/2 Rogue/2 Cleric/2 Sorcerer/5 Dragon Slayer had. One spell cast by the Wizard with 30 Int who could cast 7th level spells would be way more effective than any of the spells that same jack of all trades character had.
When none of us could find a compelling reason to play a character who didn't specialize. It very quickly became taboo to play one of these characters(though it didn't stop a couple of people from doing it anyways and just having people complain all game about it).
I mean, when the 13th level character who is only a 2nd level cleric casts a Cure Light on you for 1d8+2 and you have 10/140 hp, you COULD say "Thank you for healing me" and just be grateful that you got anything from that guy who normally keeps all his healing for himself. But it's much more likely that you'd think "Why do we keep this guy around instead of a single class cleric would could have healed me to full with one spell?"
The 3.5Ed RttToEe campaign I was in, I initially played a Ftr/Rgr/SpecWiz Diviner/Spellsword. He had few offensive spells, and was a 2Wf build with Shortsword & whip (tripper/disarmer). He saved other PCs more often than they care to admit- his divinations helped us avoid trouble; he was the only PC in the party with any ranks in Swim, which let him rescue the paralyzed and drowning thief.
I suppose that's something. Though having ran that adventure with a group of super power gamed characters, I can tell you that they didn't need any divination spells to "avoid trouble". They simply walked from room to room being so much more powerful than every enemy in the complex that they barely took damage in most encounters.
The problem was that so many of the encounters in that adventure are with classed NPCs. Classed NPCs have the weakest actual power to CR ratio out of anything in the game. So, the adventure designers(rightfully according to the rules) created encounters of EL9 to use against level 8 PCs. Unfortunately, that encounter was one level 9 Cleric against 5 level 8 PCs. They barely blinked to defeat them. Most encounters didn't have a round to act in. The others mostly consisted of me following the strategy in the adventure. It would go something like "The enemy cleric casts Bless to give himself bonuses to hit in preparation for trying to drive the PCs towards the devious trap in the back of the room. The PCs charge and each hit the cleric for...all of his hitpoints."
As for ranks in swim. Yeah, they ran into that situation as well. One of the PCs did have to jump in and save someone else. Still, better to have a character with fighting strength AND ranks in swim than one who just has ranks in swim.
My second answer is also simple: there is more than one way to do the job.
This is true. All of them are valid as long as you don't lose power by using a different method. Or at least MUCH power. If you find a way to multiclass and take feats that lets you heal as much as a straight class Cleric of your level, then good for you, welcome to the team. If you show up with a character who can heal 50 points of damage a day vs the 600 or so points a Cleric of your level is doing, we're going to ask someone else to play a Cleric, and if they don't want to, you're going to be stuck as one.
Part of your duty as a player at the table is to help all of us. All for one and one for all.
My third answer is: there are all kinds of story lines in which a group is made up of individuals of disperate power because people congregate in adventuring groups for a variety of narrative reasons. Not every D&D group needs to be Seal Team 6.
There ARE all sorts of stories that do this. None of which work as a *D&D* story. I do think that to survive every D&D group DOES need to be Seal Team 6. At least in your typical D&D game where it's 4-6 PCs who have to defeat hundreds or thousands of Beholders, Dragons, Demons, Evil Armies, and possibly Gods. You don't survive that without making sure you are the most efficient killing machines in existence.
Look at the Justice League. Batman may be an incredible character, but when it comes to massive alien invasions, he's NOT the guy for taking out starships in space. Sabotage? Yes. But not in a massive dogfight, he's not much use.
This works because the guy playing Superman doesn't have to sit in the corner while you run a 4 hour long solo adventure for Batman. And Batman doesn't have to die the first time some supervillain uses an area of effect attack that hurts Superman(instead you can use comic book logic and have him survive the blast). Instead, all you have to do is entertain the reader. The characters(or players) never have to sit around being bored. The difference in power can be written around carefully by the author.
Try playing RIFTS for a while- a game with basic classes including the Glitterboy and the Vagabond- and you can see how this can work* in a narrative to make for an excellent and enjoyable.
It used to be my favorite game back when I was 17. I started up at least 6 different campaigns, almost all of them ended with everyone having no fun. All for the same reason: The difference in power level between the characters ended up making the game no fun. A couple of sessions spent "Manning the Base" because your combat abilities amounted to "I can fire my 1d6 laser pistol" when you were up against 8 tanks while the rest of the group had Glitterboys or were gods/dragons was enough to sour a number of people to the game.
I once ran a Rifts game where I told people they should make extremely powerful characters since I wanted to run a high powered campaign. We were running late, so I didn't have time to go through their characters. They get into a battle fairly early in the game. I had it cast an area of effect fire spell at them during the first round of combat.....and 2 of the 5 PCs died. because they didn't have ANY MDC protection at all. Which didn't really fit my definition of really powerful characters, but it apparently fit theirs. One of the other players meanwhile commented that "It did only 15 MDC? You realize I have nearly 1500, right? And I regenerate. It's never going to kill me at that rate."
That degenerated into an argument about how powerful characters should be in Rifts and then we never played it again. I've asked a number of people how Rifts can be run fairly to other people who are fanatical about it. Inevitably, their answers are the same "We don't have combats in our game that aren't easily defeatable, and we rarely do combat at all. The game is all roleplaying."
It's not just combat either. Even out of combat challenges can be disparate in their difficulty that something completely impossible for one member of the group is an extremely simple task for someone else:
P1: "I'm a hacker. This lock is extremely difficult to crack. Give me an hour or two and I might be able to do it. But it'll be hard, and if I make the wrong move I'll set off the alarm."
P2: "Ugh, no time for that." Walks up and touches the door. It opens.
P1: "How'd you do that?"
P2: "I told it to open with my psychic powers."
Rifts has a great story. I loved the novels and I think it'd make a great TV show/movie. But as a game where you need to have 4-6 players play at the same time and encounter the same challenges....it never works.
The only metagame expectation that matters is that you're going to be playing with the other people at the table, each with their own idiosyncrasies & expectations of fun.
Well, it being a metagame expectation, it varies from group to group how much it matters. However, I can tell you that at our tables...it matters. If we want to play a game where party combat balance isn't important, we might play Paranoia or Gamma World or some other game we don't take that seriously and don't care if we die. But when playing D&D, one of the prime motivators of our whole group is party balance, feeling like you pull your weight and are important to the party and aren't continually overshadowed by everyone else.
Of course, the only way this works is if the game system is robust enough to make us all equally powered without trying....OR everyone agrees that we're all power gaming to the best of our ability to make sure we're roughly equal.
For instance, there is a guy in our group that plays a wizard 85% of the time in D&D, and another who plays a "sniper" just as often, regardless of system. And they play them well, which is good...but it means that most of the time, nobody else gets to explore those options. The metagame assumption we have is that it makes those guys happy, and so be it.
In our games, we'd have no problem with someone like that. But we'd want them to make a good Wizard. If they showed up with one that prepared Mage Armor in every one of their slots, including their higher level slots and did nothing but cast the spell on themselves and hide....well, they'd likely never hear the end of the complaining. And if they didn't listen to the recommendation and change their character, it'd be likely that the rest of the players would be talking to the DM and asking him if he could kick that guy out of the group because he refuses to play the game properly.
As for what is "best" much of that is subjective. Yes, there is hard math behind certain options being mechanically better...but the math is not the whole of the game.
That's certainly true. I'm positive none of the characters in my game have chosen the "best" options. Instead, they choose really good options. But they are all within an acceptable range of "best", so they don't vary a huge amount.
But, people in our group definitely notice the difference between one Fighter doing 75 damage and another one doing 10. They might not notice if someone is only doing 45 damage instead of 75. They would definitely notice the Fighter hiding under the table every combat for "roleplaying reasons".