Private dentistry is definitely worth it for dentists, with the money they charge for services...mine charges £45 per appointment, £100 per extraction or filling, and it's upward of £500 for root canals and replacement teeth and the like. There is a monthly plan for free appointments, but it only gets you 10% off treatment costs if you need anything major done. I think a major motivator for NHS dentists is probably going private, haha.
Since my wisdom teeth messed up with no warning I've been going regularly, because I never ever EVER want to go through that kind of pain again. I've never been more miserable. Even if you take care of your teeth, it definitely doesn't hurt to go twice a year for peace of mind and/or a thorough cleaning.