I've taken quite a few of them over the past three years (that's how long I've been playing). I live in Chicago where the winters are cold and snowy. Playing outdoors is impossible from about the middle of November through the middle of April, so during that span the only real option for me to get any time on the court is to take group classes.
When I started out, I took group classes exclusively. Started out once a week. Then a couple of sessions later moved to twice a week. When you're a beginner, they can be quite helpful. As you get better, you can still get something out of group classes, but only if you combine them with more time on the court outside of class, hopefully by playing with other people of a similar level. One of the benefits of the classes is they make it easy to connect with people who are about at your level, and if you can hit the court with some of them outside of class, then you'll start to see much faster improvement than classes alone. If you're able to add some private lessons on top of that, again, you'll see faster improvement.
So I don't agree that group classes are totally useless. Most of the time you get out of them what you put into them. But I do have to say that, yes, some of the drills have seemed somewhat pointless and didn't really seem to help me or anyone else improve and that felt like a frustrating waste of time.
Bottom line: I think they can be useful, but you got to do some work on your own as well. If you're just going to class once a week and hoping that's going to help you improve, then it probably won't happen.