Tourniquets can be on for a LONG time before nerve death occurs. And certainly prevent exsanguination is the top priority.
The only valid reason to NOT use a tourniquet is if you are unable to get medical attention within a few hours of application. In that case, clotting and stopping the bleed at the site is really your only hope of surviving and keeping the limb. But even then a TQ is still useful to buy time.
