I've never seen them in the U.S.A., but in Germany they are the norm. It can be a challenge to get them to release me go to a real lounge in my own chair - they're carefully tracking each disabled pax and are quite punctual.

Your US experience sounds typical - some good, some bad. I usually tip $5 for good service. Expecting more is not right.

The "stay on the plane" is the ultimate trump card. With crutches, it may be more appropriate to play it on a regular basis. With my own chair, I'm usually happy to have a crew member help me up the jetway. The only time I hold out for a dedicated pusher is where customs/immigration is involved. Which is not to say that signage indicated elevators isn't often hidden or missing.