Yeah I also prefer using mid-weights since mid-weight combos would work on lightweights too. The only time I go out of my way to select someone else is when I need to practice on heavyweight.
Up late I've been using Helena as a dummy. She's slightly heavier most of the middleweights, but all my character's middleweight combos work on her. They just require very slightly tighter timing. Unlike GenFu, whom some middleweight combos don't work on.