My airline also doesn't charge for this service BUT! IMHO I think unless you can prove you are registered disabled it should NOT be free. Everyone else should pay because believe it or not there are lazy pax out there who know they skip queues and etc when using this service, and this would weed them out.
How very true! I get really annoyed with people who are just too damn lazy to walk.
True story - I had a lady ask me for a wheelchair for her husband, they were going to Paris for the weekend she told me. I asked did she have a chair to check in (assuming he'd need one to actually get around Paris), no he just can't manage to walk to check in and the gate from the car, but they wouldn't need one when they actually got there. Turned out they didn't need one in the airport either when it turned out they'd have to wait 1/2hr. That really annoys me, because it reduces the service for those who genuinly do need the service.