Having some means to cut seat belts makes sense for rescuing others. However, I don't think there is often much need for a tool in self rescue. In the vast majority of cases, the best self rescue tool is going to be your hand, simply reaching the buckle and releasing it in the usual manner. Because we buckle and unbuckle our seat belts many times each day, this is well ingrained in muscle memory, and generally works even when disoriented after a crash. And many people remain surprisingly calm after a wreck.

The experience of a women I know well illustrates this. She left the house for work a few minutes ahead of her husband. She slid off the road on an icy spot and rolled the car upside down. Her first reaction (while hanging upside down in her seat belt) was to calmly pull out her cell phone and call her husband to tell him she was OK. He would be driving the same road a few minutes after her and she didn't want him to freak out when he saw her car wrecked by the road. She then unbuckled the seat belt and crawled out the side window, which had shattered in the crash. The woman in question was about 60 years old, reasonably fit for a woman that age, but by no means an exceptional athlete.

For rescue personel (firemen or police), a rescue tool makes perfect sense. They will be outside the car, trying to extract the person. Simply cutting the belt will work much easier than trying to reach across them to the buckle. It also makes sense for us to carry a knife or rescue tool in case we are involved in an impromptu rescue of others.