I wouldn't say it's a scam, but yes it is in many cases just pre-paid expenses. If all you need is cleanings and periodic x-rays, it might end up a wash against the cost of premiums.

I recently went through a transition from employer-provided group dental insurance, to the basic Medicare Advantage dental offered through Kaiser, and had to pay out of pocket for some major work that couldn't wait (root canal). Since then I have found an affiliation-based group plan that costs a little more but results in much lower amounts charged by the dentist for non-routine work. The quality of the dentists in the network is much higher, in my limited experience.