Hate them.
I just think they are a waste of space. I would rather have the extra edge length sharp, or have the same sharp edge length and have the whole knife be shorter and lighter.
I guess they are supposed to allow one to choke up on the blade for finer control, but the same thing can be achieved by using the portion of the edge closest to the handle, or by pinching the blade between thumb and forefinger, depending on the situation.
I’m not too concerned with the scenario you describe, though. Theoretically, I suppose a similar thing could happen with a knife that simply had a longer tang, or even with the handle of the knife depending on design. In any event, while I haven’t had to cut any seatbelts yet, I would think following up with a simple draw cut should finish the job given a sharp blade.