Two things here, and I know I usually stay out of these discussions, but:
1) There is no official Disney policy stating that people using wheelchairs/scooters should be taken to the front of the line. But, some of the queue areas, you may have noticed, do not make for easy manuevering. There would be more chance of someone getting injured trying to get around that maze-like area for The Great Movie Ride, for example, rather than having them come down the open side area which is a straightaway instead. In those instances, they may have a separate entrance for guests utilizing a wheelchair/scooter and their party. Again, this is for the safety and convenience of the guest, not as a special privelige as some people seem to think it is.
2) As far as the couple with the scooters that you saw walking: did you follow them around for an hour or two, or did you just see them for a few minutes? There are some people that have circulation problems or other health issues that allow them to walk for limited periods of time, say 10 minutes or just a little longer than that, but if they had to walk the parks, stand in the lines, and all the other things that many of us take for granted as being able to do, they wouldn't be able to make it. Just because a person shows no outward signs in the brief time that you see them of having a medical condition, doesn't mean they don't have one. A good example would be a professional golfer by the name of Casey Martin. If you don't know who I am talking about, Casey Martin was a professional golfer who, when in college, was diagnosed with a rare circulatory disorder that left his right leg weakened. He was able to walk for a limited time, but to walk the entire 18 holes of a golf course was just not possible. The NCAA allowed him to use a golf cart to ride from shot to shot. He eventually went on to join the PGA Tour and actually had to fight all the way to the Supreme Court of the United States in order to get the right to use a golf car at PGA Tour events since the rules stated a golfer had to walk to each shot. Does he ride a scooter at all times? No. Does everyone that uses a scooter when they go out use one 24/7? No, sometimes they only use it if they know they will be doing something that would be physically not possible for them overall.