A couple of things come to mind:
If you have dinner/lunch reservations at other hotels, having your own transportation is soooo much easier than transferring busses/boats, etc. But if you only have a couple of hotel reservations, you can take a taxi.
Since this is your first trip to WDW having a car can be overwhelming. The busses will get you everywhere you need to go. I think having one less thing to worry about will allow you to enjoy your vacation more.