Our firm pays part of the cost and my family has notoriously poor teeth (kids inherited soft enamel from mothers side of family) so we have used dental insurance the past several years and we have always come out ahead as a result.
To me this is one of those YMMV scenarios. If you are younger, have good dental hygiene, and have a history of being mostly problem free then dental insurance is probably not a good idea. If you are older root canals, crowns, implants and the like can add up fast.