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.
I dont know how Mainstage handles accordeon's that is not my thing..
The way i use mainstage, i have patch remain or seamless sound switching... I add all sounds i need in a song/performance in a single mainstage patch. And then i use my Kronos buttons to switch between them. They are allways live as they are in the same patch..
This takes care of 2 things... preloading times, because preloading of the next patch can be done while i play the current song..
In the end its just a matter of setting up things.. which is an option on Mainstage to archieve certain features, but is a requirement on Gigperformer.
I have never been in a situation that Mainstage could not handle..
Altough, for being really creative, i prefer using Reason as a host these days, now those combinators are a thing when combined with all the jewels inside Reason.