I think that Carlos has hit the nail on the head. When BIAB regenerates a realtrack instrument it is stitching together various segments of larger audio files together into a sequence. If it stores a note of which segments within which audio files it used and the sequence that they were used in (rather than the generated audio file itself) then it should be able to recreate it verbatim the next time the song is opened. Of course if it is not keeping a note of these items right now then it might be a reasonably big job to add that functionality, but it should be acheivable (I guess it then goes in to a prioritisation queue with other development and wish list items).

Generally speaking I have found that the degree of variation between one generation of a realtrack instrument and the next is much less than you would typically get for an equivalent Midi style (solo instruments excluded) and I personally find this greater degree of predictability better but there are still times when a particular phrase that sounded really nice vanishes when the song is saved and retreived (or sometimes a phrase which sounded just 'OK' is replaced with one that does not).

Of course sometimes the opposite happens and the regenerated track sounds nicer than the previous version, but I would prefer this to happen under my control rather than as a result of a file save and re-open.

If I hadn't seen such riches I could live with being poor.