As with any true "insurance", it's a gamble that overall the house always wins. They know the statistics, and set their prices so they come out on top. However, if you're among those who really use it, it is clearly beneficial.
If the budget is tight and there are known and certain expenses that must take priority, take the chance and skip the insurance. But if you have money in the budget, it buys peace of mind that IF you need it, it's there.
I never had a single cavity... until I hit 40. Then the bottom fell out. Don't know why. I did, and do all the things you're supposed to do. Now our dental insurance saves us hundreds, sometimes thousands of dollars every year. You pays your money and you takes your chances. BTW, some plans have a 1 year moratorium after your policy starts before they cover some of the major stuff.
If you do get it, find out what plan your dentist/periodontist use, and seriously consider that plan. You'll get more benefits and ease of use.