When I do it, its because I need the whole yeast cake for something big, and I only go from session beer to imperial, and/or mild to hoppy, because I dont want to risk carrying over more dominant flavors. I.E. I'll go from Blonde Ale to Barleywine in the same carboy, but certainly I wont go the other way. If its session to session, I'll portion it out in pint mason jars, and refrigerate the remain for other batches.