Some of the WINE programs (for example, notepad) use resources for languages that arent present in the MingW headers, the Windows SDK or the visual C++ 6 header files (like LANG_WALON). Where did these definitions come from? Does windows actually use these definitions somehow? (Given that WINE is supposed to be a clone of the windows API, defining and using things that windows itself doesnt provide doesnt make sense)
IANACWD (I Am Not A Core Wine Developer) but I would think that being able to compile the standard Wine programs and some of the non-core dlls under MingW with w32api would be a usefull test.
Being able to replace e.g. comctl32.dll from MS with the wine version and seeing what breaks would be a good way to track down bugs (IMHO) Same for Wine programs (like notepad), being able to build & use em on win32 would help with bug tracking. (i.e. is this bug in WINE or in the app)