It specifically says on Disney's website that you can't use DME as a luggage transfer service only. I've read that your bus voucher gets scanned when you get on the bus (I don't recall this happening, but there's probably a lot I don't remember) and that if you never get on a bus your luggage is left behind.
If you think about it all from a business angle, it's easy to see why they won't allow DME to be JUST a luggage service. You have to ask, WHY IS DISNEY OFFERING DME? They're doing it strictly as a business move, to make more money. But, you ask, how do they make more money by giving away a free service? Easy. If you don't spend $120 on a towncar, you have $120 more to spend on Disney property. If you don't have a grocery stop from that towncar ride you're not getting, you'll spend more on food inside WDW. I guess they're hoping you don't get a grocery delivery, like I did.
I imagine they REALLY want to compete against the rental cars. Not only will you have more money to spend at WDW if you don't spend it on a rental car, but much more importantly, if you rent a car you have the freedom to leave the WDW property and thus buy food and supplies and souvenirs in the outside world. If they can "trap" you in WDW without a rental car and without a grocery stop, they will make much more money off of you.
By the way, I don't say ANY of this in a bad way. When I go there, I LOVE being inside what I call the "Disney cocoon," where I don't step one foot off the property while I'm there. And besides, there's nothing wrong with a company coming up with new ways to make money. God Bless America.
-- Eric
