BobF,
I've used BIAB for about 4 years now. It's more sophisticated than CP but the results can be magnitudes better. If I didn't have such good friends to collab with (ohgrant, jaymesg1213, daryl1968, Beagle just to name drop a few) I could see myself producing full songs with BIAB being my collaborators.
The RealBand soloist feature (for a non soloist like me) can be a real boon to a song. Of course nothing can replace the originality of collaborators. But if you are limited in budget or collaborators BIAB could be just the thing for any composer to explore not only chord progressions but solos as well.
The only thing I will say that BIAB suffers from (if one were to use only it for tracks) is drums. It's not a robotic Casio but it's not very expressive. They say you can get the drums MIDI but I've never been all that impressed with what BIAB does with drums. I usually build my own; typically using grooves from the chosen drum VST or the multitude of purchased grooves I own.