We rented wheelchairs (manual push) for my mother every day last summer on our trip. She does not use a wheelchair normally, but has problems walking for any great length of time - after about an hour, she generally cannot walk anymore because of back and knee problems. In her daily life this isn't an issue, she just doesn't go anywhere that requires that much walking, and does things like shopping in small trips. But at WDW it was a major problem, since the rest of us wanted to do more than 1 hour of stuff a day without feeling like we were ditching my mother all day, or killing her by making her go at our pace! So the wheelchair was the answer. We DID NOT use it to skip any lines - we didn't get the disability pass - we just used it to push her around the park between attractions. She walked onto every single attraction, though the normal lines. We just made sure to fast pass everything we could, or picked things where the wait wasn't long. It allowed both her and my daughter to have an enjoyable vacation, and spend lots of time together.
So don't assume people use the wheelchairs just to bypass lines. Many who use them use them just to get around without extreme pain, and never ask to bypass a single line.