Hi Tony,
A number of people on here use RB exclusively for building arrangements without BIAB involved in the process anywhere. The arrangements that they produce are first class.
I'm one of those who begin in BB and, as soon as I have all my chords and melody working (if my song has a notated melody, that is), and I have found a suitable collection of instruments and drums that sound good, I usually jump to RB.
My reason for using BIAB first is that BIAB begins playback not too long after the generate button is clicked. This allows me to quickly hear how tracks sound. This is because BIAB begins playback before generation is complete and continues to generate the tracks in the background while playback is underway.
RB, on the other hand, requires that tracks are fully generated before playback can occur. This makes it a little slower -- especially when more than a single track is involved.
In the early stages of song creation, BIAB saves me a little bit of time.
Sometimes, because of new features in recent versions of BIAB, I'm now finding that I use BIAB entirely for generating Realtracks. I do this when I want to take advantage of the "Avoid transposition of Realtracks" option in 'Song Settings'. I've found that if I want to get the highest quality of sound, activating this option is one thing that can help.
I also like using the 'Force Simple Arrangement' option that BIAB offers individually for each track. The new 'Natural Arrangement' option offered for each track in BIAB is also sometimes useful for me. As far as I'm aware, RB does not have these features.
When click tracks, tempo maps, and unusual time signatures are important, RB is superior. Realband also allows for partial regeneration of Realtracks and Realdrums.
Just my two cents worth.
Regards,
Noel