There are many situations where a diver might be required to apply a tourniquet in the water or under it. The chances of any sport diver being placed in any of those situations is slim but not impossible. However, to deny those possibilities is denying reality. Getting a deep cut from a piece of sharp debris, a line loop snap taking off a hand, a wildlife attack, etc. are all realistic possibilities that can happen underwater and require the use of a tourniquet. Have you ever in your life watched somebody bleed out?
No, I don't carry a defib or a rescue helicopter or a Parajumper in my gear. The trick is to evaluate your current situation and make realistic choices as you what gear you will carry. I don't carry my tourniquet or my RBFK (That's R for really) for pool work but I won't dive in wild water without them. If you don't want to carry it, then don't! Hopefully somebody else will have one if you have an accident.