My reasons for not using Real Band (right or wrong) are:

1) I like the band to play variations (like real jazz band would), each time I replay a song
2) If I have not played a song for a while, I might want to change a few chords. If I am in Band in a Box, I can do that right away.
3) I can instantly change volumes of the various instruments in the mixer or all at the same time, even while the song is playing back.
4) I have not noticed any better sound quality in Real Band and I am so use to BB, that it seems to be easier for me.