You can carry a 'spare' knife your whole career and only ever need it once - but you'll be damn glad you had it if it saves your life.
I have tons of knives of all different shapes and sizes - they are tools. As I believe has already been mentioned - knives/guns/crossbows don't kill people - people kill people. Don't carry a knife if you aren't prepared to deal with the consequences of using it. Don't carry your tool that is intended for 'duty' if you aren't on duty. For God's sake don't ever carry your pocket knife to the bar when you're drinking. I've studied martial arts on and off for my entire adult life. I would rather be stabbed defending a knife attack empty handed, than try to explain how I cut down my assailant without so much as a scratch to myself. For what it matters, I learned the same lesson fighting empty handed - THE HARD WAY!!!
Anyway, before I ramble off topic for too long:
I carry a one-hand opener in my pocket for the convenience, and also during rope cutting tasks and such. After all, you can't always let go of that which you wish to cut. I've owned various types of Spyderco, an Ernie Emerson CQC7, an Applegate/Fairbairn folder, a couple of Benchmade, and others that don't rate honourable mention. I have a lot of non-clip folders too. Swiss Army, Cold Steel w/ San Mai blade, etc.......
The DH Russell JM/Engineer/Pioneer issued knife is the authorised knife for JM drills for the simple reason that it has a protective security flap that prevents accidental loss and such during the jostling and high winds during door checks etc.....plus, who wants an exposed knife brushing against their chute and eqpt as the JM works his way up/down/around the jumpers during ground and air checks? As was stated, it doesn't hold a very good edge, but hey, it's free and exchangeable.
Leatherman Wave - much better than any Gerber in my opinion. I like having the blades and saws on the outside of the handle and not having to unfold the pliers to get a blade out. Another true story: One of the my Instructors on mountain school broke his femur in an ATV roll-over. He was trying to make his way up a really steep grade to the base of a rock face and recover some eqpt. We got there and had to stabilise and evacuate by mountain stretcher. I used the saw on my Leatherman to cut down and prune the splint for his leg. Worked in a pinch - worked well.
I carry a short machete with blackie collins handle vertically mounted behind the frame of my ruck. It has come in handy over many years be it desert, winter, or jungle type terrain. The best part is it requires no special rigging for jumping. It didn't come with a sheath, so I cut down one of the issue sheaths and sealed the bottom. The clincher - if I ever had too, I would be more than happy to use it as a weapon. I'm sure it would work quite well. I think the think only cost me 15 or 20 bucks in the early '90s and it is still in service.
The Cold Steel SRK has been mentioned and IMO is an excellent low cost knife. Carbon V will rust if not taken care of, or if taken in the salt, but it can hold a wicked edge. Te best part is you can hammer and pry all day with the damn thing, you will have a hard time breaking the tip. I have the Recon Tanto also but have never had call to use/carry the damn thing.
