This is a very good question! I had been wondering the same thing.
I have everything done in standard BIAB and froze the tracks.
I was hoping the open the BIAB file in Reaper.
But it went and re-generated the tracks.
Not only does it generate the tracks, but it makes an extra copy that you need to go and clean up later.

Boring! That is not what I was hoping for. So far for me... The only use for the BIAB VST is to have the chord sheet open in Reaper.
It also help me to place the part markers.

Now I have not tried the multi riff thing yet. Maybe it will be useful.