Well, by 'the entire Wine development team' you mean Alexandre :-) ?
Not so, quite a few people work on the x11drv. For instance Alexs winedesktop work, various NETWM interop patches from me, fullscreening windows (not applied yet, but ...), system tray integration and so on. Plus all the work to make OpenGL/DirectX go of course! :)
I, for one, am happy that people are using our driver model to write new drivers as it could serve as a proof of concept that our driver model is sound (think about GL / Cairo / 'DBI engine' kind of drivers).... Heck, if we only wanted to support one driver, why do all the job of doing a clean separation and not include X11 in Wine's GDI directly ?
Well the driver model is lifted from Windows so I think it is sound. And as for why we use separate drivers, well, when the DLL separation is complete it means we can make a real Windows box display via X11 which is Just Plain Cool :)
It also allows for portability to operating systems that don't have X11. MacOS X *does* have X, in fact it's a modified XFree but with some proprietary bits added. So I'm thinking the added effort is not worth the gain. I'm not a Mac user though so who knows, maybe the extra native-ness is worth all the pain ....