I've always used BIAB to create backing tracks and then moved them over to RB for finalising. In the old days I used Powertracks but RB is just PT on steroids.

Playing live mp3s are more reliable than BIAB (IMHO) and because what i'm trying to achieve is the sound of the small band I used to play in - 2 guitars, bass, drums and sometimes keyboards - mp3s do the same job. after all do you want the bass player to play something different every time? the audience won't notice small changes and as for redoing BIAB on the fly i have enough trouble getting the guitar and vocal right without playing around with a laptop.