Disney will tell you no...and that is 'the' official word. People here, on the DIS, will tell you sure, go ahead. Will anything awful happen? Probably not.
I have to wonder why Disney doesn't do something in the way of offering bags only transport, for a charge. They would clean up. It would be simple. Your bags are scanned at the airport as they go through different areas. Your voucher is scanned as you reach the bus line. These are tied to your room, supposedly. If your bags are scanned, but a voucher isn't? Disney could add a charge to your room account. Simple. Will they? Probably not.
And I for one would prefer to see people sending their bags with DME and then renting cars vs people bringing all those bags to the bus with them, and then riding the bus. That slows down the whole deal for everyone on that bus.