You'll get lots of answers on this question. Yes in some cases, renting a car will save you a little time here and there. To us, renting a car takes a tremendous amount of the magic away. Having to drive and search for parking spaces in GIANT parking lots and also having to walk across those same HOT parking lots, both coming and going, just isn't worth it. Disney transportation, which is great, drops you off fairly close to the front entrances which is by far worth it. If you park hop, you're walking across the giant parking lot again, and then having to park in another giant parking lot. Yes, it "might" save you 5 minutes, here or there, but with as much standing in lines and walking in the parks that we do, the additional walking just isn't worth it.
Now we do book a Limo/Town Car for transfers to and from MCO and our resort. They're waiting for you when you get there and pick you up an hour and 45 min. before your flight leaves. They even make a stop for you at a grocery store if you want. This is a convenience that saves you a whole lot of time and is well worth the extra $140-$150.