I own Cubase, and can't think of a reason to sync the two. My workflow is as follows:
1. Create a test song in BIAB for the purpose of reviewing styles.
2. Once I've picked one or more styles, I develop a rough arrangement (chords).
3. I save the song in BIAB and open it in RealBand.
4. I carefully check the tempo and arrangement and make changes as needed.
5. I add tracks using various styles as needed to augment what I started with. This includes different rhythm parts, soloists, etc.
6. Once I get things more or less the way I want them I export all the audio (and MIDI if applicable) to WAV files and import them into Cubase.
7. From that point on, Cubase is the master. Before I do too much more work, I make sure that the tempo on Cubase matches the Realband project.

With this technique, I can add vocals and other tracks, but can go back to Realband and generate new Realtracks or drum tracks, tweaking shots and breaks and such as needed. All the tracks match up when I re-import them into Cubase.

I've produced my last two records (for other clients) this way, and it is a really nice workflow. BIAB/Realband make it really fast to rough things out, check tempos, keys, block arrangements, and give the talent something to sing to.

I have a few things I wish it would do - forget about getting the excellent musicians in Realband/BIAB to play a lick with you. If you are lucky you can get the drummer to accent something the way you do. And don't get me started on endings.

On the other hand, the Realtracks are pretty amazing if you don't ask them to do too much (like stretch the tempo too far or play wacky changes.)