I think John summed it up nicely.
Simple answer is BIAB is NOT a DAW! And, Realband is NOT a feature-rich, modern DAW. Realband does not compare favorably to any of the modern DAWs available today. The one thing it does is support RealTracks but when you realize it doesn't even respect BIAB bar settings, frozen tracks, etc. you see it is not even useful in your BIAB workflow!
BIAB is absolutely brilliant at generating tracks from the awesome RealTracks library. And that is enough! I get in BIAB and out to my DAW ASAP!
The ease of working in a full featured DAW is priceless. Both BB & RB have a unique thing that no DAW can easily do.... but for working with audio, it needs to be a DAW. Yes RB is technically a DAW and BB seems to be adding tracks but it's about the features and ease of use and unfortunately PG still has a ways to go to catch up to the DAWs currently widely used.