What stands on the WINE main-side for one of the new features of WINE 1.7.13? »More Windows Media Player stubs.«
So you do not only reimplementing the platform. You also reimplement the MediaPlayer, the InternetExplorer, Wordpad, etc.
A month ago, I already asked in the forum, why you don't take ReactOS programs for WINE: http://forum.winehq.org/viewtopic.php?f=8&t=20836
There are programs under the GPL. But some are under the LGPL. For example mspaint: http://svn.reactos.org/svn/reactos/trunk/reactos/base/applications/mspaint/
It can be possible, that there are programs for Windows, which makes use of mspaint and wants to start it, because it exists since Windows 95. So any Windows-developer can assume, that mspaint exists on the system. Same with some other programs.
Greatings theuserbl