Here was a post of mine from Reddit concerning this:
Tad has already said that becoming the 'best' of a specific class will be difficult and take a lot of work. I think people will still gravitate to the 'roles' they like playing, and try to make a name for themselves as that identity.
I wish I could rationally explain it, but I can't. I've only seen it, as I'm sure you have. I think there is a desire for something you put work into, to be able to be used whenever. So if I toss a ton of work into a tank, I don't really like an encounter that makes tanks useless. I've developed an identity with that character because of the work involved in making it. But at the same time, there are alt-happy people, who don't really care about really becoming a perfectionist with one role, but like being known for the person who can play anything--my wife is like that.
However, that might be a product of games where work=depth, too. So the reason we develop those identities is because games push us so deep into characters (More gear, more skills ect). Maybe a game that's built on being more fluid won't have it feel so bad when my "favorite" class isn't that great and I have to swap.
Then again, how many people tend to main one character in League of Legends? Or at most, 2-3? Even professionals, who can play everything, usually admit they enjoy or identify with one character or type better. So it's a complex thing, I really have no idea where it comes from--I do know it's sometimes a fine line to walk. Again though, hopefully in a game where it's "broad" or "horizontal" the need to attach identity to a certain class won't quite be there, if that makes sense.
Because, don't get me wrong, I love League, and I think a game like "pokemon/League" would be fun. But there are a lot of variables there that make me worry when we are talking about a persistent world.