yep, if you stay on-site you can park at the parks for free...
i think whether or not you rent a car is a personal choice... do you think you will use it? do you have special circumstances where getting from point a to point b is going to be difficult by bus? do you hate waiting and like to be the master of your own destiny? do you plan to go off-site? if so, then yes, get a car...
or... are you patient? do you find wdw transport to be part of the "experience?" do you have a lot of time to relax and take things as they come? if so, skip the car...
or, as a last alternative, maybe do a combo... both
alamo and national have rental car desks at the dolphin and car care center... we rent a car when leaving the airport (we start our vacation by visiting my elderly aunt who lives 45 mins from wdw) and use it to check in to our hotel, drop off luggage, stop at the grocery, etc... then we just drive to the dolphin and turn it in w/the valet parking attendant... take disney transport to our next destination and all is right in the world!
for getting back to the airport we've always just taken a taxi (we have always had early morning flights)... because we have a late afternoon flight, this trip we are considering going back to the dolphin (maybe after breakfast at epcot) and picking up a car, going to our hotel, packing up and heading to the airport... if we have add'l time, we'll visit a tourist trap or two along the way!
so... once you figure out what your needs are, then i think the question will answer itself
