Tourniquet use is discouraged, because of complications. This is in a civilian setting, however, where definitive care is expected to be available in short order. Direct pressure to the bleeding site is a far better way to control most extremity bleeding than a tourniquet; however, in the event of a limb amputation, and especially if I were a single rescuer with other responsibilities as well, I'd have no hesitation to place a tourniquet to limit blood loss.
As with so many other things in life, one has to consider the entire situation. Can the bleeding be controlled another way? Are there enough people present to do everything that needs to be done? How much blood is the patient losing? Are there other injuries that are more life-threatening? How far away is definitive care? A tourniquet is something in your toolbox, but it shouldn't be something that is often used.