I agree. Game-devs do this all the time -- making their own GUIs -- and they're often slicker and faster. It's not as hard making something app-specific. The difficulty is making a library that supports every edge-case that every developer needs (and still falling short).

An argument against this would be uniformity or playing nice with standards (not sure how many standards straddle the OS/Desktop spectrum though). I'm not a fan of uniformity -- it tends to constrain applications. Communication-standards are good, where we have them. But I mean look-and-feel... user-interaction should be as suited to the application. But this might be a minority opinion. I'm not a fan of stunted Apple-esque UIs, and hate Gnome's march to ever more compliance and uniformity to their own guidelines. I typically use four major applications -- what advantage does uniformity bring?