Seems to me you're making a assumption that just because someone might not wear their seatbelt, they will always be more dangerous/injury prone... If 85% of people wear their seatbelts as it is now, are you saying all of those 85% are the safe ones, and the other 15% are not? Seems to me you could have a type of driver who drives aggressive, yet wear his seatbelts, and get's in accidents. Sure, maybe he survives some he wouldn't, but if he keeps doing it, wouldn't he be affecting rates too?
IMO, you still haven't given a good reason why seatbelts need to be mandatory by law. Insurance rates go up for a lot of things..people get in plenty of accidents a year with seatbelts, as well as car theft, etc.
Again, I still think it's a good idea to wear one, I still don't see the justification for it being mandatory by law. Do the states that do not require you to wear a seatbelt have drastically higher insurance costs? It seems like they would by your reasoning... Where does the revenue go from writing tickets for non compliance?