OK - another reason. I don't get any coaching on riding form except when I've taken spin classes. Granted the ones I've taken lately have been in conjunction with MS training rides, so most of the instructors are MS-150 riders themselves, and are taking the opportunities to teach us how to tackle the hills better, or improve our riding overall. At least in spin class I can concentrate on riding form, and not worry about riding off the road.

That said, one of the worst spin classes I had was from an instructor who admitted she was a runner and not a cyclist, but had been teaching "spin" for years. It became very apparent that she didn't ride outside.