121.5 was createed for and is used for aircraft (or ATC) distress or emergency communication. It is a guard frequency monitored by all aircraft while in flight, ground stationns, and military, and is not used for general communication. Any communication other than emergency is switched to other appropriated frequencies.
Edit: Perhaps I was too short with my reply. If one has an emergency, or has a situation which requires contact with the ground, or another airplane, and cannot communicate using normal and/or other channels, then the use of 121.5 is a valid option. Safety to yourself, as well as others, defines the principle. I would never split hairs shoud the need arise.
Last edited by wes_wall; 7th November 2010 at 14:53.