OP, spinning probably isn't detrimental as long as you aren't getting overuse injuries or are spending time in spin instead of actually riding. In the case of the latter, even if it is taking time away from riding your bike, as long as you're having fun and enjoying it, why not keep doing spin unless you have some other larger bike riding goal you're aiming for?
I did them for a while and almost always had to set the resistance higher than the instructor suggested to get a hard workout. I just don't enjoy it much, so I don't do spin any more. I live in Florida, so I basically can ride outside any day of the year and prefer that.