As a guest relations cast member, I have quickly learned not to judge who truly needs a wheelchair or not. There are many guests who, based on appearance, would seem to have full mobility. However, there are countless ailments that may cause them to be less mobile than they appear.
However, I do agree with those who believe that there are those who abuse the system.
I had a guest when I worked in attractions who was missing both legs, and gladly transfered to the ride vehicle (which required quite a bit of upward arm strength). Not long after that, I had a woman who (I'm pretty sure) was quite overweight, and laughed at me when I asked if she would be able to transfer to the ride vehicle. I was very tempted to say "well, a man with no legs sure didn't have any trouble a few minutes ago, what's your excuse?" Obviously I didn't say this, because, like I said, you never know if there was an additional disability that I did not know about.
And to Merlin, who said that being a cast member isn't that great of a luxury, I'll tell you this: Keep in mind you are paying great money to have a great time. Whereas, I am being paid to have an even greater time! (granted, the pay may not be that splendid)