I agree with OP. I don't think every game should have a custom character, I wouldn't want a Witcher game with a custom protagonist, but I think GTA's perfectly suited for it. For one, I don't think the story would particularly suffer, because I don't think GTA tells particularly good stories anyway. I definitely wouldn't want to replace Niko Bellic, he's cool, but the main thing I care about in a game like GTA is the freedom of the crime sandbox, and I think that would only be strengthened by being able to customize my character.