dinabean,

the most of the spin class instructor are not cyclists, they are aerobic teachers. Generally in the real biking world, we don't spend a lot of time standing, only for short bursts and maybe uphill for getting off your butt from the saddle for a very short time.
I see 99 % of the members spinning on a very high cadence and bounce like a ball on the saddle, bad postures and so on...there is so much more incorrect stuff they teach, just for entertainment, well I can get this somewhere else and way better... that's the reason I cut down on classes from 5 time a week to 1 class. I just don't want to waist my time anymore in classes like this, I bike outside and if the weather is bad I spin at home on my trainer. If you get an instructor which bikes outside, you will see a difference in class quality.
So in other words, you decide what you want to do in the spin class, this is your time and most important it is your health!