This is a touchy subject here, but I think there is a big difference between an onsite guest parking at a different resort, and an off site guest parking at a given resort. I'm pretty sure they only tow the latter. If you are an onsite guest, you are allowed to park at any other resort. If you are an offsite guest than it is true they will only give you a three hour parking pass.
Having said that, in most cases, it won't save you time. In a few cases it might be easier, but in most cases it won't be, and you'll be inconveniencing other guests in the process. The bus sytem is pretty good. Let's say that you park at the Contemporary to go to the MK, you'll have to walk the extra distance to your car, when it's easier to take the bus back to your resort. It might save a few minutes of waiting time at the bus stop, or bus time while the bus drops off other guests, but usually the wait for a bus isn't very long.
If you drive to Epcot, and you go to the EMH hours, you might find yourself at the far end of the park at the end of the night. During Epcot EMH, most of the attractions are in the future world part of the park. Most of the WS food pavillions are closed. So you'd have to walk all the way to the far end of the WS, then all the way around the BWK/BC/YC, then all the way out to your car (unless you pay for valet service). (The Swan/Doplhin now charge for parking.) Since you aren't a resort guest, you aren't going to get a spot near the hotel. (We've gone to the BWK at night for dinner and evening activities.) Plus, for the drive home, you have to stay on the public roads. The buses have special access roads, so your drive home will likely require more driving. WDW is not the easiest place to navigate. You are going to be tired, and you'll have to concentrate. If you miss your turn, things could get ugly. Even if you do everything correctly, it's going to take you a LOT longer to get back to your hotel than simply taking the bus. AND last but not least, the designated driver has to remain sober.
From MGM to BWK for parking, you'll also have to take the boat from MGM to the BWK area. The wait for that boat is at least as long as the wait for your bus would be, and you'll still have to walk out to where your car is parked at the BWK. So again, I don't see how this is any easier than simply taking the bus back to your resort.
Now, if you are planning on eating at the GF, say, and you're worried about getting there on time from your onsite resort, then it might be slightly faster to drive directly to their parking lot. But again, it might actually take you longer to get back to your resort at the end of the night. Or if you were staying at Pop, and you wanted to eat at CSR then go to a park, it would be easier to take a car to CSR, and then take your car to the park.