Spin class standing climbs taught me the mechanism of pedaling around the stroke at both heavy and light resistance. There are hills around here that I wouldn't be able to ascend at all without that skill, because at <30 rpm one dead spot in the stroke would topple me! And I can jump out of the saddle in any gear even at high cadence. Pretty sweet.
Spin class seated climbs taught me how to scootch around on the seat and find the leverage to really haul that pedal around at high resistance. And how to get the beat of the music in my head and spin my cadence up even if my legs are complaining.
Cadence drills also taught me to keep my legs spinning when the difficulty increases. And showed me what spinning 120 without bouncing feels like.
When I sprint in spin class, I give it my ALL (unless they're doing multiple minute "sprints", then I just go really hard). Recovered or no, I go HARD again for the next sprint. A lot of people can't motivate themselves to get their heart rate really high in class, but I feel fortunate because I can get a killer interval workout. Sometimes my HR doesn't drop under 180 after warmup, now THAT'S a good class!
When they do jumps, I try to *explode* off my seat for every one, making it a plyometric type drill and training for quick starts in a race sprint or just jumping off the line at a traffic light.
I think spinning is good for all kinds of cycling skills! But it will never substitute for riding.
It's my off-season sad replacement.