For anyone supplementing arrangers with backingtracks, I can highly recommend the Onstage Performer. Here is the link: http://www.playwareinnovations.com/
Ok, it does not play midifiles as of yet, but what I have done is recording or rendering the files to wav. It is such a userfriendly programme that allows you to enter lyrics(identify song parts with lyrics), build sets(when you pick a song for a set, it is removed from the main list for that set), edit songs, eq on the fly and the list goes on and on. This is the product that made me return the midjay since version to of MJ couldn't handle long songs with text synch. That is now fixed in version 3 of MJ. However, this is a product worth looking at. No, I don't get any money from saying this. I just wanna spread the word as I did last year.
I now have quite a few VST and DXi plug ins that I fix my midis in Sonar. The improvement over any standard KB and soundmodules are quite impressive. I also replace many drumtracks in midifiles with loops from Drums On Demand: http://www.drumsondemand.com/
Needless to say the realism is striking.