I'm an EMT. When I'm on duty I carry one of those little, orange-handled Spyderco Rescues, the 79mm, serrated, blunt-tipped version. I carry it in my right front pocket because my department prefers that we keep our knives out of sight, and out of reach, of potentially irrational patients (a policy which I personally think is quite sound).
I agree that a knife is not strictly necessary, because a shears will do almost as well, most of the time. However, I know that if I have to reach one hand deep into a wreck to cut a belt, or if I ever have to disentangle somebody from a rope or a harness, I'm going to wish I had a small sharp knife instead.
I selected this particular knife very carefully. It is light weight, flat and rounded, pocket-friendly and reasonably priced. It opens with one hand, locks firmly, stays locked in use, and is surprisingly sharp and strong. I did modify the blade ever so slightly, rounding the very tip of the cutting edge off with a stone to make the knife just a little safer when used close to human skin. I added a small braided lanyard to make it easier to fish out of my pocket when I need it.
I have yet to use if for anything important in the field, but I'm always happy that it's there. And yes, it works great on boxes, too...