not necessarily. most plans seem to be pretty bad at covering major work, but every plan I've had has paid out more for my two yearly cleanings than I pay in premiums, making it a no brainer.
Yeah, basically. Most dental insurance I've seen has a "benefit maximum" [0], the maximum amount the plan will cover in a 12 month period, I've seen numbers from 1.5k to 3k for this. Since the coverage is capped, and capped so low, it's arguably not really "insurance", and isn't really protecting me.
That said, to a graduate student, $3k can be a non-negligible amount of money, and the school is likely kicking in money to pay for the plan. In my position, if my company didn't subsidize the plan, I'd be happy to just go without.