I know this will sound awful, but we love having the option of a grocery stop so that we don't have to pay $7 per drink for the entire trip. It's nice to have the option of making your own drinks in your own room, and then taking them to the pool. Or having a nightcap at the end of a long day to just mellow out and relax.
As far as using DME (which I know isn't really the question, but you can just play the opposite game)
Pros: It's free. It gets you where you need to go. No need to worry about luggage.
Cons: You have to be at the airport/set up your pickup time about 3 hours ahead of your flight, whereas with a towncar you can put it a bit later or to your preference. You have to deal with other families and the potential of them being late to the bus, which is why the 3 hour time frame. You don't get to stop for drinks/snacks at the grocery store. You have to wait for your luggage to appear in your room, if you wanted to immediately freshen up and go when you get to the room you really are giving up that option, unless you want to rely on what's in your carryon. Watching advertisements for the Disney Parks feels a little more forced on the bus, personally I didn't feel like it was getting me more excited to get to my location, it only made me excited for the film to be over.
I'm not gonna lie, I'm a bit biased, but I've used rental cars and Happy Limo and I really prefer those options to DME. The personal time preference is probably the biggest factor that makes the decision for me, along with having my luggage with me. At elast if it's lost I can find out at the airport, and not hours after I get to the resort.