I'd be interested to hear how many piots leave mode 'C' turned off even if they have it fitted? I know for a fact that it's not that uncommon, quite often asking the pilot 'G-xx, do you have mode C?' will lead to it magically appearing.
What I don't know if these pilots appreciate is that if they are in class G and not talking to anyone, (no problems with that, btw, it's up to you), with 7000 set and no mode 'C', they are effectively wiping out a 10 (ten) mile diameter cylinder of airspace around them from the surface to FL245 for anyone who is in receipt of a radar advisory service.
The radar controller providing the service must achieve - if the subject of the RAS cannot see the unknown traffic - either lateral separation of 5 miles or 3000 feet vertically based on mode 'C' returns from the unknown aircraft (our non-talking 7000 squawker). The easiest of these is nearly always the 3000 feet, and this is not an option if there is no mode 'C'.
The most frustrating thing is when the unknown aircraft is showing on secondary radar only since it is too low for primary contact. As a controller you know that it is down in the weeds, but the rules say that you must take a 10 mile detour with your departing passenger jet which is passing FL100.
So does anyone leave their mode 'C' off as a matter of course? I'm genuinely interested.