I do take your point about interferance with operations in other FIRs. Since the majority of "practice pans" are carried out at low level, I don't think this is much of a problem. If you are at FL350 in the Paris FIR in an emergency, I would suggest that 121.5 is probably not the most appropriate frequency.
121.5 is monitored by D&D cells at Scottish and London Centres both with autotriangulation equipment. Does anyone know whether similar triangulation is used by other countries? Just interested to know.
G W-H