It's the DMs job to juggle it so everyone has a chance in the spotlight, and everyone has an opportunity to contribute.
I heavily disagree with this. If I'm running the Dungeon of Ultimate DOOM that I've been spending the last year writing up in great detail filled with horrendously deadly traps and nasty monsters who want to eat your face and you show up with a character who is an 8 year old girl who spends her days picking daisies in the field and doesn't know how to wield any weapons or solve any puzzles...you should expect to have nothing to contribute to the party...and likely to die.
D&D isn't improv acting. It's a specific game with specific conventions. One of which is "You will face nasty monsters and you need to be able to defeat them".
The person you were replying to specifically mentioned Pathfinder Society adventures, at that. In Pathfinder Society, the DM isn't allowed to change the tone of the game or adapt to a non-combat player at the table. They get an adventure that says "You enter the room and the door slams behind you. In the room are 3 beholders who want you dead. Roll for initiative!" That's what you have to run.
If your games are so skewed to combat that someone who creates a character who isn't good at combat won't have fun - it's on the DM to address that, not the player.
No it isn't. You make a character that fits the game you are playing. If I'm playing a Call of Cthulhu game, I'm likely not rolling up a Space Marine or even a Commando. They don't fit the game. Most of that game involves running away from enemies and avoiding touching books. Playing a Commando with Automatic weaponry is missing the point.
D&D is mostly about defeating monsters, venturing into dungeons, and solving problems that require larger than life abilities to solve.
That's not to say that there aren't people out there using the D&D rules to run games that aren't within the D&D theme. I'm certain there will be 100 people who read this who tell me I'm completely wrong and that D&D doesn't HAVE to involve monsters, combat, dungeons, or even problem solving.
Be that as it may, D&D should support that as its primary method of play.
In fact, a thief with lots of thievery and social skills is a GREAT character for the DMs game. This offers the opportunity for an excellent cat-burger adventure, or a political intrigue adventure, or all kinds of great adventures! Why would you not like that, as a DM?
Because most of those adventures don't work well in a group. A cat-burglar adventure reminds me of the time I attempted to run a rogue sneaking into a castle as an adventure. None of the rest of the party could help because they were so bad at stealth. So, I went into the other room and ran the PC through an adventure where they hid from guards and made their way from room to room searching the castle. Which took 2-3 hours. With 5 other players sitting in the other room being extremely impatient that they weren't being involved.
Political intrigue always reminds me of the adventure I once played where one player spent the whole session attempting to make political contacts and woo one of the ladies in the court while the rest of us sat there looking at our 8 charisma scores and decided not to open our mouths to avoid causing any problems for a couple of hours straight.
Combat, puzzles, and exploration get the entire party involved. Everyone has to help defeat the beholder. Everyone has a chance to figure out the puzzle. Everyone can contribute to searching the room.
Not that you can't use those other things as pieces of an adventure to allow one person to shine. But you need to keep them short enough that no one else gets bored and quickly move onto the parts of the game everyone can contribute to.
So, if someone is ONLY good at the parts of the game that tend to exclude everyone else, they don't make a good character.