Milesofthejungle

I recently bought logic express9 as an upgrade from garageband and am now thinking of upgrading to logic pro, mainly to get mainstage. I currently use 2 keyboards in a live situation (mostly due to inability to swap patches), but figure I might be able to cut down to one using midi with mainstage for splits and swapping between patches using the software 'midflight'. Is this the sort of thing it's useful for? What would I use to play backing tracks? Can they still play out of logic, while mainstage is open, or would I have to use itunes? Couldn't I just use logic without mainstage to do all this?