Setting aside for a moment the "ethical" question, when I have heard people using backing tracks, it has tended to sound thin, tinny, and extremely cheesy--the exception being in Casino floor shows, where there was dancing involved, and a full scale theatrical sound system was used--though even in these situations, a live musician or two(often percussionists) are often used to give the sound a bit more depth(though the cheesiness doesn't disappear)--

"live" music has an energy level that it that is noticably missing from canned music--even if you are just a singer accompanied by a guitar--if you're not loud enough you can always turn up the volume--