"Those who can, do. Those who can't, teach."
- Some Worthless Teat Who Probably Couldn't Do and Couldn't Teach
Sherman Chavoor, famed swimming coach of Mark Spitz and numerous Olympic champions and world record holders, was not a good swimmer, nor a poor swimmer. I'm sure he would have looked great in a Speedo, but the fact was that "Sherm" Chavoor could not swim AT ALL. Although many great coaches are former athletes, many of them were, at best, mediocre performers. Why is it that many great coaches were not even close to great performers? And why is that many great former athletes make crummy coaches?
Often, great coaches are people that found out how to perform (and instruct) better through experimentation and close observation. Conversely, many great athletes have had, comparatively, few roadblocks to their success - their efforts, for whatever reason, directly netted them gains and their progress was relatively linear. A coach or athlete that has had to dig and scrape and search for his or her successes may be better able to offer constructive advice for a struggling athlete. A "natural", on the other hand, may flippantly say "What's wrong with you? Just do it!" - and, for them, it makes perfect sense.
Recently, I read a message board thread taking a noted Strength & Conditioning coach to task for his poor execution of lifting technique. Watching the video, I agreed 100% with the criticisms of the coach's less than stellar form. The lesson to be learned here is that a great coach does not have to be (nor should they try to be) the model player. Many of my best coaches relied heavily on video (or film) of great athletes to show proper technique - in fact, aside from the martial arts, I don't remember a single coach ever "chalking up" or "suiting up".