Up to this point, RealTracks have all been single instruments. However, there have been requests for instrumental sections, such as a horn section, strings, or vocal "ooh"s and "aaah"s.
If BIAB supported such things, it would be great if these tracks could be individual tracks, instead of single track. Additionally, BiaB could export each instrument to an external DAW as a separate instrument.
The main reason for doing this would be to allow more control of each instrument in the mix. For example, the pan and volume of each instrument could be controlled independently.
It would also be helpful for the folk that want more control of drum tracks as well, assuming that each drum part was sent to a different channel.
I imagine that with some cleverness you could also mix-and-match instrumental lines, although that would probably get complicated pretty quickly. For example, you could have a string quartet with each instrument having a static line, and a couple variants where each instrument played a more interesting line with passing notes. Basically, sort of what BiaB already does, but since it's in a single instrument, it would be possible to coordinate the results better - for example, only allowing one of the instruments to play an embellished line while the others played a held chordal.