Some enjoy complexity for complexity's sake. Some enjoy trying new things. And some enjoy looking through a dozen books and pulling from each to create the character that they have in their head.
While it can be taken to an extreme, players shouldn't be automatically dissed for doing this.
At the end of my friend's campaign, I had a dragonwrought kobold spirit shaman with draconic feats(as per sorceror) and half the spirit shaman abilities traded out for more 'draconic' stuff.
Was it because I wanted to powergame? No. It was because I wanted to have a kobold shaman who channeled the powers of his tribe's dragon, while slowly becoming one himself. I went through several classes and systems to get there (warmage, sorceror, Elements of Magic mage, spirit shaman, some Iron Heroes stuff, Complete Arcane's draconic feats), but all of it was in service to the character concept, not powergaming.
If I'd wanted a powerful mage character, I'd just have rolled up a grey elf wizard...