DME is NOT an automatic service. It must be reserved in advance. You can not just arrive at the airport and expect to ride the bus, or to have your bags delivered.
It must be reserved before you go. It takes a bit to get the packet sent out. But, here's the thing. There is no way I would pay for a towncar service but tag my bags and allow Disney to transport them. Why? Because if I'm paying $100+ to get to WDW, I do not want to wait up to 4 or so hours to get my bags. I want my bags with me.
Yes, I tag my bags and allow DME to move them for me. Why?? I'm not paying for the service...not handling the bags is one of the reasons I use DME rather than pay for a car service. But once money is out of my pocket, I don't want to have to wait for those bags. I am not willing to be inconvenienced if I'm paying, out of pocket. I will be inconvenienced if I'm not paying.