I teach spin classes, and I'm also a cycle coach, and deliver Wattbike training.
It's far too easy to get 'qualified' to instruct a spin class, and some of them are really really terrible. The worst thing is when they tell you to 'dip' over your pedals, i.e. come off the saddle, and bring your bum low near the pedals whilst pedalling - if an instructor tells you to do this tell them not to be so bloody stupid - guaranteed to cause undue stress to knees, possibly with serious consequences. Other gimmicks like press ups off the handlebars are less dangerous, but equally pointless.
Problem is many instructors want to make the classes 'interesting', and ensure that you feel like you've worked (you certainly feel it in your legs if you're stupid enough to do the dips). Personally, I make my classes hard, very hard, we end up with a ratchet climb. 30 second intervals - half to one turn on, then quarter to half a turn off, so with every interval you're increasing resistance, up until you struggle to hold 60 rpm (I don't believe it taking cadence too low or too high).
Bottom line though is that spin classes can be useful, and also you can just do what you want, if you enjoy the context and the music, then you can pretty much do your own thing. Maybe speak to the instructor and tell them that you're a cyclist and you'd like to do your own thing, if you're going to be deviating a lot from what they say, and pick a bike at the back of the class.
If you can find a spin class on wattbikes, or Keiser spin bikes with power, then I'd recommend that for getting a lot more structure, especially if you get the Wattbike app and can analyse your data and pedal stroke afterwards.