It does seem like an exceedingly rare occasion to dedicate an entire part of your knife design to...
If I needed to cut my own seatbelt, I'd get it done, even if I cut myself a bit in the process.
But for rescue workers, I can see how it could be useful.
I've cut quite a few seat belts over the years... usually after cutting the roof off of a vehicle to get people out. (Think about where the shoulder belt feeds through and you will realize why you you need to cut them away to remove the roof completely.) At that point you are cutting well away from the person and you usually have quite a few tools, shears, etc. to choose from to cut the belt.
I don't ever remember cutting one because it jammed though.
However, where they are extremely useful is cutting away a person's clothing with less risk of cutting their skin. Shortly after getting the Kershaw Funxion EMT, I was carrying it at work to try it out. We were dispatched for a person who had a heart attack while riding their bicycle. Their heart had stopped, and we were performing CPR on them. I needed to open their shirt and attach big sticky pads to look at their heart rhythm and possibly deliver a shock. As, it turns out it worked great. And we were successful in saving a person that day.
Benchmade rescue hooks work great for the same thing also, and I have used sheepsfoot blade knives (Leatherman Super Tool 200 serrated blade comes to mind) if needed also.
Ideally, I'd love a 915 Triage but I'm hesitant to carry something that expensive at work and lose it. Although, I did pick up a 940-1 as my first BM for casual carry....
Back on topic, seat belt cutters are not necessarily Taticlol, but are often misunderstood / marketed.
Honestly, I've found most rescue marketed tools are questionable at best though, with the exception of the above mentioned tools, and the Gerber Hinderer Rescue (Although not great it's functional.) And I've got high hopes for the LM Raptor, but I haven't used it at work yet.
If not obvious from the post, I'm a career Firefighter (13+ years) / Paramedic / Instructor