Again, if the DM has no problem with my PC, what business is it of yours what I'm playing? In LotR, they didn't have a test to see which hobbit(s) they thought could make the trip.
My answer to this is simple: When I'm relying on you to keep me alive in a dangerous situation, it's my business if you are capable of doing that job. My life depends on it.
Suboptimal characters certainly work in some games. Mostly, in games where the DM isn't trying his best to make really hard encounters.
But in most of our games, our DM is looking at it from the point of view of "I don't pay 100% attention to the numbers on everyone's characters, it would drive me crazy to keep that all in my head." So instead, they estimate the amount of damage the party puts out by min-maxing in their own head(20 strength to start, so someone at level 12 has 23 strength, with a +6 stat enhancer, so 29 strength, they have a +5 weapon, have +4 more damage from feats, so they have a minimum of +14 damage, with buffs up, likely +20). Then they pretty much take that as "average" damage" and then try to find monsters that can survive a party of 6 each doing 25 damage twice per round(i.e. 300 hitpoints worth of monsters or greater so they don't die during the first round, likely 600 to 900 hitpoints of enemies so that it lasts a couple of rounds).
Then, when you have people in the group whose combat effectiveness is WAY below the optimal, people start dying because of the difference between expected power and actually power.
Oddly enough, it's almost never the suboptimal PC who dies. They sit there round after round missing entirely, or hitting for 1d6+3 points of damage, or hiding under a table. Meanwhile, the powergamed Barbarian is hitting for 75 damage a round, so the DM has all the monsters focus him. Then, some round of combat comes along where the monster is at 10 hitpoints and the Barbarian is at 10, and knows that when he drops out of Rage, he'll be very close to dead. But don't worry, the enemy doesn't go next and everyone in the group can deal 10 damage minimum. Except, the person who goes next is the suboptimal character. He misses due to low attack bonuses...or even more disappointingly hits for 8 points of damage. Then the enemy goes, hits for 30 damage on the Barbarian and kills him.
At that point, the player of the Barbarian is frustrated, he loved his character and didn't want to die. Does it become his business that you've chosen to play a character that is purposefully bad at combat? In fact, wouldn't it be proper roleplaying to test members of the group to make sure they could hold their own in combat before they joined? If you are going to rely on people to keep you alive, it would only be prudent. But that doesn't happen because no player wants to have their character kicked out of the group before they even get to start playing. So, mostly via social contract we HAVE to accept new characters into the group, no matter how bad they are.
In LotR, they didn't have a test to see which hobbit(s) they thought could make the trip.
No, they didn't. But then again, they didn't have much of a choice. They just started following them. I haven't read through the whole book, but I believe there was definitely concern brought up about bringing a bunch untrained people with them on a really dangerous mission. To the point where they felt the need to train the hobbits to a suitable level of combat training so they could hold their own and wouldn't need to be protected all the time.
Plus, there's a severe difference between a book with a bunch of characters in it and a game being played by players. Characters are fighting for their lives and most people who aren't very good at fighting wouldn't be caught anywhere NEAR a fight...or they'd be practicing every day to get better because being bad means death. Plus, often you don't have a choice of your companions. They lost their family and you need to protect them. The fact that they don't know how to fight isn't their fault. And the author of the book can simply write about how these weak characters miraculously survive. They were never in any danger, the author never intended on them dying.
Meanwhile, when you are playing a game, there's a bunch of metagame expectations that go on top of the normal game. Each player makes up his character using the same rules and the same options. You know there's going to be 4-6 of you and that whatever characters they make, you'll be stuck with them because you are all playing in a D&D game that you've agreed to play weekly. You know the DM is going to throw challenges against you and you'll have to work together to defeat them. It only makes sense that if the point of the game is to work together to defeat these challenges that everyone should try their best to help in that goal. Choosing options that are really bad ON PURPOSE is going against the cooperative nature of the game.
Sure, in character, that thief who lives on the street has never been in a fight in his life and he got swept up in some plot to kill the king and is traveling with a rag tag group of adventurers who are beset by challenges they've never knew they'd have to face. However, out of character, you knew the DM was going to be throwing those challenges against you and one player decided to play a Rogue with no combat ability making the chance of the group surviving those challenges lower. That's kind of jerky.