Yes, you get to park free...nice perk of staying onsite. Most people will suggest that driving to the parks isn't a bad idea. But, parking at MK isn't a great idea. You have to park, possibly take the tram, unless you get a front spot or don't mind the walk, to the monorail or ferry. Take one of those to MK. Versus taking the bus that will drop you a very short walk to the park entrance.
I've had a car at WDW...and it's stayed in the parking spot at my resort almost the entire time. I just don't mind the buses. I enjoy talking to people and being on the bus lets me do that. I'm usually not in a huge rush to get to the park.
But many others dislike the buses, and love having their cars.