It's not just ATC who have a problem if Mode C isn't used.
The pilot of any aircraft with TCAS, (which detects aircraft with an operating transponder), will not know the altitude of the other aircraft without C.
This means that his TCAS will give a loud "TRAFFIC! TRAFFIC!" voice warning (20 seconds to a collision, impossible to ignore) even when the aircraft may be well separated in altitude. I can't stress how difficult it is to locate an aircraft visually if you don't know whether it is above, below, or at the same level as yourself.
This causes a major problem to passenger carrying aircraft. The last thing I want to do is to start any unnecessary hard manoeuvring, it puts the $hit$ up the pax, that's what TCAS is supposed to prevent. It also greatly reduces the number of heartbeats left over for my retirement, if you get my drift.
If you've got it, please use it whenever you use the transponder. It's for your safety as well as mine.
I do my best to avoid you, so please help us both out.
Thanks.
