Comparing with other physical activities. If you ride a horse, if you do ice skating or anything else. Are you expected to take private lessons, if you are serious? Around here, for sure not! That would be pretty unheard of, unless you are elite. Sports are always taught for free here meaning that teachers/instructors/coaches do not get paid, except on very elite level, and dancing is to certain extent regarded as a sport.
My guess is that there exist different "cultures", so to speak, where privates are necessary or not. Group classes are probably taught differently if serious students are expected to take private lessons as a complement, than if they are supposed to teach all the students need to know. There are ways of teaching classes in other ways than to the lowest common denominator; every martial arts teacher I had did that! I doubt that dancing is that more advanced than martial arts... although I suppose it can be different in different kinds of dances; I do not know much about about ballroom but from what I have heard you need lots of technical correction so I guess maybe squirrel is right about privates being a must there.
For dancing, I think we have two kinds of places here - the non profit sports club, and the dance studios. Maybe some places are kind of inbetween. I decided to take classes at a place that functions pretty much as an average sports club, in that sense. Maybe someone thinks that this automatically means low standards, but I do not think so. A pretty impressive list of competition results say otherwise! I voted that privates are unheard of in my surroundings; at least I did not hear about it but possibly it exists among those who compete.