And when you have the training, you don't need to carry one. Field expedients are readily available.
Well I wouldn't say that's 100% accurate but I get where you're going and don't really disagree. And I would also add that I'm no expert on field medicine, advanced first aid or any similar field... Which is one reason why I don't carry one of them or any other advanced first aid supplies. I've had some basic first aid, BLS and AED classes and have enough appreciation for what I've learned to know that there's more that I don't know than what I do.
I think for people with advanced first aid training, or any specific training covering tourniquets, having a real one ready to go is much better than having to improvise one with less than ideal circumstances surrounding you. If you don't have that training, it'd be like carrying a gun but not having any honest idea of how to use it.
I read some study many years back about the use and misuse of tourniquets. It was pretty interesting. Lots of ways their use can go pear shaped. I've never felt that, for me personally, it was worth the time and effort to really put in learning about their use. Hopefully that won't ever come back to bite me but I guess if things are ever that bad, I'll just try what I know and hope the rest works out.