I don't remember exactly what, but When I tried out Mainstage with my midi accordion, I couldn't do some simple tasks that I could do in other VST host apps. Many MS users seem to like and use Gig Performer
http://forums.musicplayer.com/ubbthreads.php/topics/2823169/5.

Another thing is, is there what Roland calls patch remain in Mainstage? It's an important feature for me, and it works very well in GP.