It's a huge chunk of time wasted. You are much better off parking in a park lot, and then taking park to park transportation when you change parks.
Here's the problem.....the buses heading to resorts don't really get ranking until 9ish. So, you get to DTD, park your car, and then trek to the bus stops. There's 20 mins. Wait another (possibly) 10-25 mins for a resort bus to arrive. Then a ride to the resort (15-25 mins). Then, depending on the park you want to go to and the resort you're at, you may be able to walk to the park or take a boat, bus, or monorail. If you chose a particular resort, lets say the CR,to get to a particular park, lets say MK, you may find yourself waiting longer for that particular resort bus to get to the DTD stop.
Those that use DTD to park and get to a park, are spending a lot of time doing so. And those that are doing it to save on parking are throwing away park time!