I think having your own transport is a must - even if staying on-site I would want my own car.
Couldn't imagine just being on WDW for the whole holiday so to just spend a few $$$ on taxi's just wouldn't work for us , it would just be disney overload.
For trips to the mall, supermarket , the other theme parks such as Universal which we enjoy just as much as WDW.
Guess it all depends on the type of holiday you like to enjoy, but just being able to go where you want, more importantly when you want is such a big advantage and it gives you the freedom to leave Orlando too.
I can see the advantages of using Disney transportation, certainly will be making full use of the monorail system for our Polynesian stay next year but....we still have a car.
If you shop around I don't think it is that expensive either, for a minivan we are paying £300 for 2 weeks, inclusive of all taxes (not taxi's!)
I would do that every time.
I guess also we have been lucky to have explored more of Florida in general, certainly many of our earlier holiday's were the old-fashioned 2 centre type which included the likes of Fort Lauderdale, Miami, Naples, Palm Beach, Sarasota and the Key's - so with that we have become confident in exploring further afield, I guess the car is much a part of our holiday as the theme park's.