Interesting point Speed Freek. Whilst flying a jet with TCAS two or three years ago, we passed a Cherokee no more than 2 nm away at the same height who was definitely squawking mode C and nothing appeared on our display. (I verified this with the Cherokee's pilot on the phone since he was known to me).
However, other light stuff has appeared on TCAS displays I've used with or without mode C read outs.
Strange?