The kneeling buses are also great for parents with strollers! I've seen the older handicapped lifts in use. They take up a lot of space on the bus and took a long time to operate. No one wants to make themselves that conspicuous, and obviously it's too much trouble for just a stroller.
I think it's a good idea to kneel the bus for every stop. Many people have handicaps that are not immediately obviously. I have a good friend, for instance, who has a connective tissue disease although she looks young and healthy - stairs are a big problem for her. (She gets dirty looks on elevators.) She would never ask to have the bus knelt down, but it would certainly be easier for her.
It's also safer for everyone if the bus kneels down - less chance of tripping, etc.