Originally Posted by Rustle
If everything was squawking there would be issues on a radar display.
The benefit of S over C is selective interrogation, so that when the day comes (31/3/2008) that everything is squawking the system copes.
Which is why it makes so much sense to have mode S for all of the new transponder users (microlights, PFA, glider, hang glider, parachutes, etc.) - except for the fact that current technology and cost don't really work for many (any?) of these.
It is also why replacing a perfectly good Mode C with a Mode S is so annoying. And even more annoyingly the dis-information and emotion have people who are equipped for mode A only not spending the extra couple of 100 quid to have mode C, and people with mode C not using it! - all taking away a proven effective system of reducing collision risk and creating some quite scary near misses.