I used to do back to back classes if I could get them, because I needed 2+ hrs on a bike for my training. Usually, these classes would not be during peak times, so the 3 or 4 of us who did it weren't taking bikes away from anyone else. During peak hours, if I needed more time, I'd use a spin bike in the general rec areas, if there was one at that gym.

Anyway, as for the people who really shouldn't be there...I have no idea. I've seen people bring their own music into classes. I saw one woman bring a book. I see plenty of people only do 30 min (of a much longer class) and then leave. I guess that's ok if you have a tight schedule some days and want to squeeze in a workout, but not when some people were shut out of the class who would've stayed the whole time. Sometimes, these are classes not covered by membership (but cost extra), so I wonder why they wasted their money...