I have limited experience with MIDI but the vast majority of MIDI music I have heard all sounds like the circus just pulled into town (no offense to circuses is intended)!

So when I was checking out BIAB the main thing I was interested in was RealTracks.
For me, the benefit of BIAB with RealTracks is I can sit down and within minutes get a complete accompaniment that sounds fantastic. Maybe, as Bob said, it would be possible with MIDI if I purchased a great synth and then spent the time to learn to play MIDI but that kinda defeats the whole purpose for me. If I have to learn something in order to make a great guitar track I'd rather learn to play guitar than learn to play MIDI to sound like a guitar!
So, currently I'm resigned to taking what RealTracks gives me and doing some mix & match editing in my DAW to get close to what I want. Or playing the part myself if I get desperate!