Without passing judgement, I think most gamers can think of at least one or two people they've known who clearly played idealised versions of themselves, or used their PCs to act out their fantasies.
It doesn't necessarily make for bad roleplaying, either; one of the best immersive "actor" roleplayers I know tends to play variations upon a constant theme, which is a loose version of how he sees himself. His PCs tend to be people struggling with problems often engendered by their circumstances of birth or other elements of their lives beyond their control, and trying to rise above those problems in one way or another. He definitely has a "type", but he's great to play with.
I've seen people miss an opportunity to learn something about themselves by examining how they roleplay; they unconsciously exhibit many of their insecurities about themselves and their abilities not only in their characters but also in their attitude towards the game.
When you have characters who express their players' insecurities about social interaction by being Charisma-monkeys always trying to dominate social situations, coupled with the player's attitude towards their character showing extreme defensiveness - an inability to distinguish between the reactions of PCs towards their own character and the reactions of players to the character (or to the player!) - it's possible that self-examination can be a positive force for change.
Had these players realised how much they revealed about themselves through their PCs, they could have learned something about themselves and possibly made an effort to banish their insecurities. Instead, in nearly all cases, the negative reaction of other players to these players' passive-aggressive defensiveness and lashing out meant that they elected to quit playing - blaming their own supposed stupidity, inability to create "good" characters, or lack of roleplaying skill for their perceived "failure" - rather than confront the root causes of their experiences.
I'm sure this sounds heavier than I intend it - in truth, the people I play with aren't self-analytical Freudian gamers, I'm just that damn perceptive.
Still, even the conscious decision to play someone different entirely from yourself implies and requires the ability to take honest stock of your own personality and deliberately try to avoid replicating your own habits and traits in your character - and this is a skill that can be developed through reflection on what elements of your past characters might have been unconscious expressions of your own personality.
The alienist I mentioned upthread was characterised by his (to me, mildly) disturbing thought patterns and behaviours. However, a great deal of his personality was based unconsciously upon my own habit of trying to provoke reactions in others through "shock" humour. In the context of the character, this element of my personality was transformed into a visceral strangeness, but part of the enjoyment I derived from the game was in playing on his "alien" nature to provoke some of the same reactions in other PCs and other players that I do in real life with over-the-top comments.
The cavalryman I'm playing in a d20 Wheel of Time campaign is brash and overconfident, and I'm grateful that my mechanical character optimisation skills are good
enough that he's often effective enough to justify his arrogance. While I don't have a great deal of overconfidence in real life, it's possible to recognise in his behaviour some of the thoughtlessness that characterises my own actions from time to time - the main difference is that he's reckless in combat, where it's a lot easier to emerge the clear victor, whereas my own similar behaviour tends to take place in more ambiguous social contexts where it's not only harder to succeed but also to recognise success clearly; accordingly, he's a dick about his abilities, and I rarely am.
That's not to say, however, that I consider either of these characters to be an idealised projection of myself. On the contrary, I doubt I'd like either of them if I knew them - and would I ever run away from the alienst, if only because of the people he knows!
Nor do I make exaggerations or alterations of my own personality traits the focus either when creating characters or playing them. Whatever insight I can derive about myself from my roleplaying is something that occurs during infrequent reflection on why my characters act and think the way they do. It almost never comes to mind before or during a game - only after.