I think I'm correct in saying that in the UK the only Centre-based agency which is allowed to monitor 121.5 is the Distress and Diversion (D&D) cell at London and Scottish. They have the autotriangulation facilities and necessary links to ground transmitters and receivers to ensure coverage down to quite low level in most areas. (They also monitor UHF 243).
As civil controllers, we are not even able to select 121.5 on our frequency panels without being given a code by D&D. I suppose it is logical that such a function should be concentrated in one unit only, since controllers have enough to do controlling on their own frequency and any of their traffic having an emergency will report it on the sector frequency first. We do get a lot of reports of ELTs which the aircraft who monitor 121.5 hear, and we pass them on to D&D as appropriate.
If EVERY aircraft were mandated to monitor 121.5 (providing they have a second radio) then it would make getting in contact with yet another american pilot who has gone AWOL from the sector frequency for the last 100 miles a lot easier than it is at present..!
In summary: Agree that it is best left to the agencies designed for the purpose and aircraft who have the will and equipment.