I never really cared that much about my system being OSS or not, I just want it to run, to run well and to behave like I want. One could argue that fglrx doesn't really do that, but actually I almost never have any problems with it (apart from starting a second X server or the non-functional compiz blur plugin).
About getting Wine to run on full OSS systems... It's just not practical. How long has the OpenGL 2.0 specification been out? We still don't have GSLG support in OSS drivers, how are the Wine D3D guys supposed to implement Direct3D 10 support that way? It's not that we didn't care about OSS drivers by the way, but for most features the OSS still need to catch up first.
And to the sound backend topic... I can also just tell from my personal experience that I never had any problems with sound in Wine (it was just the intel ALSA driver which doesn't support software mixing correctly, but that's another topic).