That's all well and good but in certain niches you need custom graphics. A good example is gaming, have you seen how beautiful the blizzard client is? Steam is nice too. Users of these apps don't care about a unified experience with the rest of the win32 ecosystem, in some ways these apps are a different ecosystem unto themselves.
I was trying to write a poker client so same thing, using normal buttons just wouldn't have made any sense. It would've been ugly as hell.
Modern UIs stray from a standard because standards are boring and if your app is boring then your not so boring competitor will snatch your users.