If you look at it and say "that's a lot of blood" and direct pressure doesn't control it damn quick, or of it's bright red blood coming in spurts/with force then it ok I'll's time for a tourniquet. (Watch a video on youtube if you're not squeamish, the US military makes great ones) A tourniquet CAN cause an injury or loss of limb. Uncontrolled major bleeding WILL lead to death in quick order. If you're comfortable carrying a tourniquet, then go for it. The guidelines have changed on this in the last couple years at least where I work to allow tourniquet use in more situations. New guidelines also allow for removal of the tourniquet in the field while watching for bleeding, if it starts again tighten it up but if it doesn't then awesome.
Mechanisms for a deadly bleed don't have to be massive and catastrophic. A friend of mine (a doctor I might add) buried a knife in his thigh getting his lunch ready with his son on a ski trip. Maybe you catch your leg on a ladder and tear an artery, maybe something else happens. Tourniquets aren't too expensive, don't weigh much, don't take up much space and can/will save a life.
Now let's talk about chest decompression needles!
(None of this is basic by the way. First aid boils down to if it's wet keep it wet, if it's dry keep it dry, splint it if it's broken and when in doubt apply direct pressure)