I work out the chords and basic style in BIAB and then I import into RB and try out various Real Tracks & Drums.
Then I import the wave and MIDI files into my DAW (Cubase 5) where I can edit, cut & paste to my heart's content.

I usually try out a bunch of different Real Tracks & Drums, but I can't do any midi tracks except whatever is in the style.
What would be fabulous in RB & BIAB is the ability to generate a MIDI track in any style and insert it into the song.
Obviously this would work better in RB because it has more tracks to work with,

Sometimes, there is a part in a style that I really like, but I like other elements in another style.
I know you can create your own "combination" styles, but what I am looking for is the ability to audition
say, MIDI Bass styles, till I find one or two I like in combination with my Real Tracks, etc.

You can do this already. Generate your song with whatever style you want. Then, either copy the track(s) you want to keep below the BIAB area of RealBand, select another style, and regenerate. The tracks will regenerate based on the new style. Alternately, after you generate tracks based on a style you like, you can also highlight individual tracks (either the whole track or parts of the track), choose a different style, and regenerate. Only the highlighted portion gets regenerated. Lots of ways to skin that cat.

If you only want to listen to the bass track, then either mute the others, or solo the bass track to hear just that.