(I'm sure you all really wanted to know that...)
In regards to using MC as a front end, I don't see what the issues are. If you have the PC, with WMC on it, and tied to the car's electronics correctly, why wouldn't it work? (I'm assuming Win7 w/ MC supports touch screens... if it doesn't, obvious problem).
I can easily see the lack of plugins for logging/ecu, and some of the more car-specific features (not that you couldn't write the plugins, but...), but basic media functionality should work.
There are SDKs for WMC as well, you've got to know .net instead of java, but you can do a lot.
With that said, I think the custom/open-source community generally avoids MS development for many reasons... and there is a LOT more customization available there, then in the MS space... and you've got a ton more support (ie community).