I haven't encoded any MPC files yet, except for test ones, and the benefits of MPC are:

1) Extremely high quality audio (Suitable for archiving and transcoding if lossless is not an option)
2) Very fast encoding time
3) More efficient bitrate usage: MPC has a lower bitrate than MP3s with comparable audio quality. (BTW MPC is VBR only)

If it's based on MP3, it seems likely that it uses techniques covered by the MP3 patents; if so, and bearing in mind that the patent holders would rather people wanting an upgrade from MP3 chose MP3Pro, it seems very unlikely that this will ever become licensable for the car-player. All the BSD source code in the world won't help that; if anyone's interested in MPC support on the car-player, they'll need to approach their country's legislature to get software patents ruled invalid first.

Otherwise, use Vorbis; it shares at least advantages 1 and 3. Or FLAC.

Peter