Before ME, there was very little (if any) additional cost for renting a car versus paying for roundtrip airport transfer.
We visited in June but stayed at Universal Studio area hotels for our first few nights then stayed at Port Orleans-Riverside.
Even without a visit to a non-disney park, we would have rented a car. We make visits to the grocery and eat outside of the parks on occasion. We can save enough in one offsite meal to pay for a day's rental on a car.
The car is so much more convenient than Disney transportation in most situations.