Can you elaborate? I know for a fact that many people fly with Mode C switched off intentionally. Could be because they were trained that way (there are a lot of old attitudes going around in GA)
Certainly. If you are flying around outside controlled airspace without mode C on, nobody knows what level you are at. You could be at 500', you could equally well be at FL 240. Your groundspeed can give a rough indication as to how high you might be, but it's just a guess.
Now, along come London Mil working an E-3 AWACS at, say, FL 200. Outside controlled airspace, like you, and a big expensive aeroplane with lots of people on board, so probably under a Radar Advisory Service. Could equally well be a civil unit working an airliner inbound to or outbound from a regional airport outside controlled airspace, the problem is the same.
Say the big jet needs to pass through the same chunk of airspace you are in, in all probability much higher than you, and in reality no danger of a confliction. The controller providing the RAS has to achieve (or do their best to achieve) EITHER 3000' vertical separation based on your mode C or 5 miles horizontal separation. Ideally, the controller would prefer to take 3000' vertical separation above you - however, if your mode C is turned off, the 3000' vertical separation isn't an option, and the controller has to take 5 miles horizontally against you. Effectively, you are wiping out a 10 mile diameter cylinder of airspace centered on your aircraft, from the surface to FL 245, that an aircraft under RAS has to be vectored around. I understand that some transponders are mode A only, but I cannot for the life of me understand why someone would intentionally leave mode C off. It happens a lot though.
or perhaps some do it because they can fly higher without busting CAS...
I would question the sanity of someone who did that. Just because no-one knows thet are doing it, doesn't mean they aren't busting CAS, or heading for a nasty encounter with a 747 for that matter.
Hope that explains what I meant. Clear as mud, probably
Edit: Whoops, it wasn't me that you were asking to elaborate. I feel a bit daft now.