As a family that always drives, here's why we use our car.
Over the course of the entire trip, it just saves time.
Using the busses/boats/etc. are great if you want to go to one of the parks; there are buses that go directly there. But if you want to go someplace else on Disney property from your hotel, it almost always involves a bus transfer.
On a previous trip, my husband had a seminar to attend elswhere off site. We had driven down to Disney from NJ, but were staying at AllStar. He took the car to go to the seminar because we thought "No problem, the kids and I will just use the busses to go play mini-golf in the morning, return to the hotel after and then we'll be together to hit the park in the afternoon". Let me preface by saying that the day before, we had gone to Fantasia Gardens by car from All Star and it took about 10 minutes to drive.
I found out I needed to transfer at DTD to get to Fantasia Gardens from the All Star. Long story short...between bus travel time and waiting time it took my kids and I 90 minutes to get from All Star to golf. I couldn't bear another 90 minute trip back, so I asked the CM there about a better way. He told us to walk over to the Swan/Dolphin, take the boat to Epcot and then grab a bus back to All Star. Faster? yes...only an hour.
Since I knew it was 10 minutes by car, I was disgusted that a little over 2 hours of my vacation was wasted away travelling.
Monorail is great, the occasional bus or boat fine. But the majority of the time, we prefer our car.