I'd go with a business rational over the theory of distributive just over lifespans. It's an adverse-selection problem. Wisdom-tooth extraction is usually not an emergency (Nick's case excepted!)-- they have to come out sooner or later, but often you've got some time. And it's a big cost compared to most dental work. If the insurer covered all or most of it, you'd have people signing up for dental coverage, getting their teeth pulled, and dropping out.