I've seen the bass thing done before as well. In fact, a friend of mine has a band that traditionally had two guitarists and when one left he didn't get a replacement, he just recorded all of that guy's guitar parts and they played along to the track live! So there was one guitarist on stage with two guitar sounds. That's a bridge too far in my book.
Tonight I'll be bludgeoned with backing tracks of keyboards for Paradise Lost. Oh well, at least I know what to expect. I hope they don't play anything with female vocals because if I hear a backing track chick who isn't there I'm going to lose it.