I have to say in my opinion aircraft squawking Nil Mode Charlie are more trouble than if they switched their transponder off completely. The reason I say that is the number of calls I take from aircraft in my airspace getting close to these (hopefully) low level aircraft, and being obliged to find out who they are and what level they are at, due to TCAS. When my response is I don't know who it is and I assume they are below CTA, there is a very nervous "Roger" from the jet pilots.