Personally - and I hate to say this as I have MANY working musician friends....but I like canned music for a lot of reasons.  Reason number one is the cost of hiring enough pieces to have a good full sound when hiring a lot of musicians would cost significantly more - even based on BASIC union scale pay.  THEN you have the canned music at the beginning of the production, start to finish, it never takes a break, never gets sick, never is late, never is OFF key because they're having a bad night, never complains, drinks NO coffee, flirts with NONE of the women in the show, doesn't smoke funny cigarettes, doesn't require food, pay, cold drinks, ashtrays, pats on the back, has no ego, no expectations, no attitudes, etc., etc., I COULD go on but you get the drift by now I hope.  I like it because while it does put a lot of full time working musicians out of work often, it DOES provide for a smaller company who could never afford a full orchestra the chance to have one at a radically less costly figure.  Couple that with all the things I said above and it's really hard to beat.  GRANTED I LOVE LIVE orchestration - makes a much warmer, richer, in-your-chest kind of feel - but who can afford them anymore!!  I know that the singers onstage HAVE TO KNOW their part as mistakes cannot be covered by a responsive orchestra but it's theater karaoke - and I KNOW that's reviled by many people.  But in a pinch when needed and nothing else can be afforded, I opt for that.  I've used canned stuff at least a half dozen times or more on shows and the only complaint I ever get is from lazy actors who don't do their job and prepare.  BUT they have the music from the beginning to learn with, rehearse with, practice with, it CAN be altered in key, nuance, timing, cuts, vamps (have to count them) and so forth.  Often they have to "fit" the dialouge into the amount of time they know they have in the canned vamps and such, but that's just a matter of getting used to it.  PLUS with midi files, vamps can be added, amended, altered, shortened (rarely a need to shorten them) or whatever it takes.  Volumes can be balanced because it's pre-recorded so a good sound guy and a decent mixer is about all it takes....or you can run it right out of a computer as I've done numerous times.  Nice just to sit there and punch the ENTER or RETURN key on a computer or with a mouse click START or whatever you need done. 

This has nothing to do with the pro's or con's of canned v's live Muso's question.

With the exception of the very best, a "PA" system is a poor excuse for a hi-fi, and canned music is seldom very engaging when heard through such a system - it can even be a distraction.  The remedy is to have the canned music in multitrack form, so that each track can be processed and mixed in a manner that makes the most of the sound system at hand.  I doubt that such tracks are readily available.