Let's say you want to get to MK early, for rope drop maybe. You'll have to be at DTD by 7:30ish, then try to get a bus to the CR....and the buses to the resorts just aren't running all that frequently. Why?? No need to.
Or, you want to just get to MK during the day...park at DTD, walk over to the resort bus stops (10 mins), wait for the CR bus (could be up to 30 mins), ride to the CR (about 15-20 mins), then walk over to MK (8-10 mins). Is saving $15 worth at least an hour of your time?
To get to DHS, you have to go to BW (or other Epcot resort) and then either boat or walk to DHS....15 min walk (peppy pace) or take the boat (close to 25 mins). Or to Epcot, it's just a 10 min walk.
AK??? Bus to just about any resort and then transfer to the AK bus.
Or you can just walk over to SSR (about 15+ mins) and take their park buses.
BUT....that is frowned upon. DTD parking is for DTD business. Just go to a park, pay to park, and save your receipt. You only pay once a day, no matter how many parks you go to. Makes much more sense I would think.