In all my years of travelling to WDW (not as many times as a lot of you, though) I have come across quite a few people that park at DTD for free to visit the theme parks. I honestly don't see how this saves them that much money. They stay at a hotel off of Disney property and drive their car to DTD; then they take a bus to another hotel and then ANOTHER bus to get to the theme park they're going to for that day. What a total waste of time!!!
Unless I'm mistaken, once you pay to park your car at one theme park for the day you don't have to pay again even if you drive to another theme park in the same day. I remember two years ago all of the cars parked at DTD Marketplace and there wasn't that many people there shopping/eating at 10:00 a.m. Later on that day we walked over to look at SSR (the DVC we own at and haven't been able to stay at--long story) and saw (or rather heard) why there were so many cars at DTD...park for free at DTD and ride the SSR buses for free to the parks.
Now, I like saving money as much as the next person but TIME is money as well. Why save a few bucks when it's going to take you longer to get somewhere? Do I like paying tolls on a highway? No, but it saves time.