I think the medley feature was one of those "...see what we can do" things that PGM has put out there over the past years. Nice for a user who whats to hear a lot of different soloists stepping in and out of the mix, with out much knowledge or care for arranging the song themselves.
F5 is one of the best features in BIAB which will give you the ability to set up who plays what and when. But of course you have to want to do that.
Edit, I see Charley has pointed out a potential for good to come from this feature. So there is always an upside.