Everyone is different. We only have our car with us when we drive (20+ hours) to WDW. When we fly, we do Magical Express and do not rent a car.
Over the years we have driven our car to the parks at times and also relied on a Disney transportation. For the most part, we prefer to just use Disney transportation.
For us, renting a car is an added hassle, especially with little kids. I’d have to fly with both kids car seats (rental ones are not guaranteed to be available, and from experience, can be in questionable condition.) Then get everything at baggage claim, shuttle to the rental place with bags, 2 giant car seats, a stroller and 2 little kids. By taking Magical Express, we don’t need to pack/check car seats. Skipping baggage claim is great. We just hop aboard the bus. We fly in early, so our bags are always at the resort by the time we get our villa. Just so convenient.
We don’t go off site at all, so we don’t need a car for that. When we’re at Disney, we’re at Disney.
I actually like Disney transportation. Buses, monorails, boats etc. We drive all over the place all week at home, so it’s nice (even when we have our own car) to have someone take us around all week while we are on vacation. I’d rather use the money we’d spend on a rental car for a Minnie Van budget instead. They have the 2 car seats we need (new and in excellent condition) and are great for us when going between locations that are tougher to get to via bus (for example, going resort to resort.)
Rental car isn’t for us, but everyone is different!