Donald, for the most part, you're correct. EMS personnel will use EMT Shears rather than a knife. The reason for the knife is (at least for me) that my turnout gear didn't allow for a lot of tools. Space was at a minimum and I felt that a knife would be more versatile than a pair of shears.
It's been my experience that seat belts don't really jam but rather it's too difficult to hit the seat belt's release from the driver's side while working on a patient. The car's a mess anyways, and rather than reaching over and below a patient to hit the release, I'd rather just cut it wherever I could. In a situation that was not too serious, I've gone around to the other side to hit the release as well. Depends on the call.
As for clothing removal, shears are best, but if someone codes on me when my shears aren't within reach, I've used whatever knife I had to cut the shirt/bra away from the person. Just a matter of expediency.
Also, many EMS personnel will arrive at the scene of a motor vehicle accident without their equipment, other than what's already on their person. If I see an accident, I'll go to help even if I don't have my other equipment on hand. I ALWAYS have a knife though.
Hope that helped,
VG