I think a lot of people import BIAB songs is because they were BIABers before RealBand came out. I have several hundred arrangements done in BIAB and if I need more control over them, they can easily be imported into RealBand.

BIAB will allow you to try a bunch of different things without too much effort. You can change styles, drums, etc and hear the result by just hitting play. For live performances, BIAB offers plenty of features such as the conductor that allow you to change things on the fly. RealBand is not for that. It takes your arrangement and 'sets it in stone'.

RealBand gives you much more control over how your arrangement sounds but involves more 'hands-on' from you. BIAB does most of that for you.

Hope this helps,

Rachael