While Mel's point about 121.5 being monitored by airliners and military aircraft, there's another larger point that is very germane to this discussion...
An overflying airliner hears your 121.5 ELT and calls it into the rescue coordination center via whatever flight service station he's using. This results in an initial search area of about 250-300nm radius. Do the math and you'll find this wonderful ELT report has resulted in a rather huge search area. Yes, we're now looking for a needle in a haystack.
By contrast, when a 406MHz ELT is detected by satellite it produces an entirely different result. First detection is via the geostationary satellites (GEOSAR) which doesn't get position information but does get the unique ID programmed into the ELT. So we now know exactly who's ELT is transmitting, and this kicks off a communications search. Often this is as far as things go because the ELT owner is reached on his cell phone and he replies that he's sitting in his hangar and has just messed with the ELT and set it off. End of search, end of false alarm.
If things go to the next stage, in a few minutes a low-earth-orbit satellite will pass overhead in its regular orbit and by using doppler technique it will provide a position estimate on the transmitting ELT. That position estimate is typically 3nm radius or better. That's quite a bit better than the position estimate from the 121.5 ELT heard by the friendly airliner pilot.
Now ask yourself... If you were injured, moderately incapacitated and the OAT was dropping quickly, which would you rather your rescuers were dealing with - a 300nm radius search area or a 3nm radius search area?
I know what my answer is, and that's why I've been 406-equipped for over a year.
