Hi Narayano,
Hope you are well.This is an interesting question in the sense that it appears to me for whatever reason that Biab generates better backing tracks (for the want of a better word)when you generate the backing tracks from a "midi melody" than of the chord sheet.It sometimes appears that at least a feel of the melody is in the backing track when you use a midi.
Forum member icelander discussed this last year and the thread below might help you out urs Hugh
http://www.pgmusic.com/forums/ubbthreads.php?ubb=showflat&Number=417858&gonew=1