In addition to what everyone else said, I recommend Gwern's "Console Insurance". Also, Jacob from Early Retirement Extreme says the following about dental and vision insurance:

I don’t have dental or vision insurance. Paying insurance that covers “regular maintenance” like teeth cleaning or contact lenses which these kinds of insurance do makes no sense whatsoever. Suppose everybody pays $25/month for contacts. Now do you think that everybody paying those $25 through an insurance company will make it any cheaper? No, the insurance company will add a $5 administrative fee—they most definitely will not give away free money. As such this kind of insurance is nothing but a financing plan for people who can’t figure out how to save the money for a $200 dental visit. The point of insurance is to cover rare events with a six-figure cost, which dental or vision simply doesn’t have.