There's a limit, or diminishing returns, on performance improvements through focusing on a main. It depends greatly on the individual. Some players can, and do, play one of each class with high proficiency without degrading their main's performance. Others are totally screwed trying to swap back to their Warrior main after playing their Rogue alt.

Ultimately it boils down to what players know and don't know their limits. I would say that more players overextend themselves than don't, but those that have a bunch of alts without doing so are great assets.

Actually I think playing a myriad of alts really can strengthen your main's performance (and in the interest of full disclosure, I am an unabashed altaholic -- with 4, nearly 5 80s and about six ot seven 70+).

The reason I believe this is, let's take your average rogue. I'm using a rogue as an example because a rogue is a very pure class -- he can't heal, he can't tank, he can only DPS and only from melee.

Now let's have this rogue roll a tank. The rogue suddenly has a whole new perspective on the game -- that of a tank who has to actually WORK to hold aggro, and how that's done. The rogue, assuming he's competent and actually strives to learn about his new class, now understands how to be a good DPSer a lot more and how to help the tank and not pull aggro from him.

Things like that... switching your role around, seeing things from another role's shoes, can really make you a better all around player, just because of the perspective.Even if he really stinks at his tank, he's still come out of it a better rogue.

Just to clarify my comment, I agree having alts is a great way to see other classes, specs and rolls (tank, healer, dps) in the game. Playing those alts will bring knowledge to you as a gamer to play better on your main character. BUT, for that knowledge to really benefit you in a raid, where you and your character need to be at your best, you need to have a main character that you as a player focus on - i.e. keeping up with the latest theorycrafting for the particular class and spec combination of your main character.

When I think of a true "alt-aholic" I think of a person who has a lot of characters in various degrees of leveling (even potentially lots of 80s), but none of those characters are that player's main character, and so all of the alt-aholic's characters are left behind in comparison to players who have a true main character

Using Snuzzle's example, I would guess the main character is the rogue. He/She is using his alts to learn about other rolls in the game to make their rogue the best it can be. And if I had to guess, I would guess the websites and blogs that are most visited by that person pertain to rogues (maybe even sites that specialize in a certain spec: Assassin, Sub or Combat).

My particular issue, from a raid-leader's stand point, involves alt-aholics who just level characters to level characters; they don't consider any one of their characters their main character and don't spend the extra time on any one of their characters to really maximize it's potential, and therefore none of their characters are raid-worthy.

I have friends and guildmates who are like this. They're great for running heroics with, but none of their toons are able to handle a progression raid like ICC. Unfortunately for them, this is why they are left on the bench week after week.