I worked on an ortho unit where it was very common for people to faint -especially their first time up after surgery. We kept an ammonia inhalant tped to darned near every doorway and most of us would keep one in the pocket. You would just know the look on the patient's face once they started moving and knew they were going to go out on you. The ammonia inhalant if administered at that moment would more often than not snap them out of their near syncope and physival therapy or whatever was going on would usually be able to conitune with no problems. Even for patients that have a full fledged fainitng spell - many would take much longer to snap out of it in they didn't have the sensory stimulation. Granted, i know - ammonia fumes can be toxic, but the amount that an inhalant puts out is very small and the idea is to pass once or twice under the nose, not bathe the person's face in fumes.
Is it necessary for every patient? - probably not - like you said, fainiting usually resolves itself in a few minutes. I would not use an inhalant for a patient with a suspected head injury (i.e. patient that has fallen and hit head, person knocked unconscious playing a sport ir during a fight), but for a garden variety fainting spell i think it's not a bad tool to have at hand.