I'm kind of half and half. We've now travelled to WDW so many times that we no longer feel the need to be 'trapped' on property and like to go do other things which a rental car makes much easier. I also have some local friends that I'll often meet for dinner at some point during the trip. As well if we have a rental car we find ourselves going off site to eat more often. This saves money compared to eating at Disney. But we don't rent a vehicle every trip, or even for the whole trip. We balance how expensive the rental is against the benefits we would enjoy.
Then as stated above, having a rental car normally allows faster transportation at your own control. We can pop out to
Walmart to grab something we missed that isn't sold at the resort (happens so often!), or to grocery shop. At the end of the day when I'm exhausted, I don't have to worry about my urges to strangle screaming children or fighting adults. I don't have to stand on my feet for 20 minutes while the bus takes me back to the hotel and my hip is killing me because I've been walking all day, but I'm not visibly disabled so nobody is willing to give up their seat for me (I have an ongoing SI joint issue that can be quite painful).
The benefits of renting a car are quite extensive but it all depends on the way you vacation. If you never leave property, or can deal with the drawbacks of Disney's transportation system then you probably don't need one. Everybody can make their own choice!