Current training is that you never, ever use tourniquets, unless it's clear that someone is absolutely going to bleed out without it, especially if it's on a limb that is clearly mangled beyond saving or totally missing. They won't even talk about tourniquets in first aid training anymore except to say "do not do it."

So if there is clearly massive arterial bleeding, I'd do it if pressure wasn't working, otherwise no.

EDIT: Actually, having done some reading, I'll amend that. Throw out the tourniquets, pack a 50 gram Quickclot pad. The 50 gram size is said to be able to stop a femoral artery bleed. I'm buying some.