BIAB is amazing, and many of its subtleties have to do with the same fine tuning that can be done differently in RB. Since I prefer fine-tuning in RB, I find that I use BIAB mostly to generate my chords in a particular style.

In other words, I enter my chords, pick a style and hit PLAY/GEN and the whole song gets built

How much trouble would it be to add that capability to RB?

You can regenerate all the BiaB tracks now (for the whole song). BiaB tracks are the blue ones at the top that appear after opening a BiaB file in RB.
'Generate' button is located in that middle section of the screen, it offers the option to regenerate all BB tracks .. which is the same functionality as BiaB.

Hey cool!
I've formed the habit of generating one track at a time in RB, and I guess it never occurred to me to try that

While it is true you can regenerate the entire song in RB...it is very slow compared to BIAB.... that's why I do my initial work in BIAB...settle on a style import into RB to add or subtract intruments and then import into SONAR X2 because of the effects like Melodyne...however I sometimes do cut the vocals in RB to use the Harmonizer there for background singers.

To me, it would be a HUGE improvement if RB could regenerate the melody as quickly as BIAB...I don't have the technical understanding of why that isn't a simple addition to RB...If that could be done I probably would start everything in RB

you have hit on the essence of what I tried to say so badly

The reason BiaB generates faster than RB is that BiaB starts playing before it fully generates the whole song. RB waits till it is done, since it was designed as a multitracker, that is not an exactly what BiaB is.