the benefit to doing it that way is that you end up with a less "chunky" feel to the finished quilt. the blocks/sections of the top are sewn only to each other, as in the traditional method. the back is all one piece (or pieced) as in the traditional method. it's only the sections of batting inside that end up different. once you trim and join them, the result feels and acts the same as one big piece of batting.
i've done several quilts this way. having the batting "in the way" does make piecing the top a bit more labor intensive but, overall, it's a great time saver.
i still put something underneath the batting, though, so i don't end up with big blobs of lint in my bobbin area. i've used super light-weight muslin; fabric i don't like any more; cut up clothes that are too worn out to donate; old t-shirts; whatever comes to hand. i've also used pages from oversized junk mail flyers and brochures. the paper is so thin it tears away easily when i'm ready for the final stages of assembly.