Every time this topic comes up I just shake my head. We rented a scooter for my dad on every trip we took to Disney in the past 4 or 5 years. He always had trouble steering the thing in large crowds, you talk about people getting run over, how about the people who walk along in a daze and step right in front of a moving scooter? Or the people who stop dead in their tracks with someone behind them riding a scooter? Or for that matter, pushing a stroller OR a wheelchair! It's not always the drivers fault, and yes they can be intimidating to drive when you first start out.
We took our last family trip with my dad in December, he passed away suddenly and unexpectedly in January. As sad as I am to have lost him, I'm so happy that we had that last trip to Disney with him. I'm glad that I rented him a scooter for the entire trip and I'm glad I never had to worry about him over doing it.
As for why people don't use wheelchairs instead of scooters, I think it's a big load off a family member's shoulders not to have to feel like a burden who has to be pushed around in a wheelchair. And no, my dad would not have been capable of pushing himself around.