Hi Nick, If you buy BIAB you won't regret it, though it does have its fiobles like all software,
Check out the real track demos you can get such sounds instantly and its really true that you can change any chord and get a very decent result. Micro tweaking is possible in MIDI but not so in Real Tracks which are Acid type loops.
For most instruments MIDI is never going to hack it, for my ears - no matter how good the samples are this is because of the natural noises and blending of notes that most real instruments make. I could go into detail but maybe another time.

If you only use BIAB fro practice its gonig to be your best friend, and its giggable too if you wish.