I think it is important to remember that a spin class is not a cycling class. Too often I hear cyclist slam a spin class because, 'you would never really do that on your bike'. Sure, that's true. There are also things that are done in a cardio-kickboxing class that are never done in kick boxing, or in cardio-tennis that are never done in a tennis match. Spin is a group exercise class that has elements in common with cycling, but it is not cycling. Some instructors are cyclists, some taught other group exercises before getting certified to teach spin. I am not a good enough cyclist to comment upon whether it helps your cycling in any significant way, but I know that in an hour long class I sweat buckets and can burn off 550-700 calories depending on the instructor and how hard I push.
Brina
"Truth goes through three stages: first it is ridiculed; then violently opposed; finally, it’s accepted as being self-evident." Schopenhauer