It seems to me everyone always assumes that a player either is a "true roleplayer" who has a character concept and tries to make the rules fit it as best as he can, or a "powergamer" who finds the best character build he can and makes up a roleplay reason to justify it. Why can't someone be both?

In one of my campaigns, a player found a race in some monster book... I honestly don't remember where... with a small shapeshifting class. After reading over that class, he came up with the idea of playing a squirrel sorcerer. He was from another plane, and was previously the familiar of a powerful wizard. The wizard died doing some kind of magical experiment, which in the process gave his familiar some inherent magical powers, and shunted him to the material plane.

So he played a squirrel sorcerer, who initially didn't know he could use magic (until he saw someone use a spell, he didn't try it himself... not optimal, as his spell list was self-limited to things that the party wizard or NPCs cast in his presence). He accepted a level adjustment to do this, and the first time he used his shapeshifting powers (quite a way into the campaign), the players (not to mention their characters) were genuinely surprised.

He went by "Tweleve" because the first thing he did in this plane was walk into a bar, and overhear someone saying "It's Twelve O'Clock." He assumed they were talking about him, so that must be his name.

Yes, he had some mechanical advantages being so small, and having shapeshifting abilities. But none who's utility could not have been easily emulated by low level spells (reduce person and alter self). But he gave up casting progression to get them. It was a mechanically inferior build that was inspired by the fiddly mechanical options of third edition D&D.

This is why I love third edition. The mechanics can inspire richer roleplaying as much as roleplay can lead to mechanical choices.