How do you introduce a new PC to an existing group? Is it best if the GM takes special measures of some kind to integrate the new and existing characters, or should the GM just allow them to meet and let the players put it all together? Does it matter whether the game is oriented towards character cooperation or character competition?
A lot depends on the game that's being run.
I have just introduced a new character for an old player in my D&D 3E game. The player had decided that he didn't want to play his original character (that'd he'd been playing for about five levels, and over a year of real time), so I was stuck with how to introduce a new character.
Fortunately for this game, I have enforced a built-in backstory. Everyone must owe the same NPC a debt, and he has called it in. So the players have an opportunity to develop a good backstory to deal with the debt, and it gives the characters a reason to be journeying together.
I think it is best if the GM works with the player to devise a way for the new character to be accepted within the group. I definitely don't think it should be left entirely up to the player and players. I've seen a few cases where the player brings in a new PC against the GM's advice, only to have the rest of the characters hate or extremely dislike the new character, and won't travel with them. The player needs to be aware of this...I understand the need to play a "real" character, but if you're designing someone that will rub the rest of the characters the wrong way, then why bother? Why bring in a character that won't last more than one session with the group?
So leaving it all up to the players is not a good idea, in my experience.
As for cooperation vs. competition - I've never really done a long-term game with character substitutions. I did a competitive "throne war" type game once, and it was limited in scope. WEF was somewhat competitive, but more cooperative, really. I introduced a number of new PCs into that game, but always worked with the player to develop a backstory that would hopefully help them fit into the game.