CAS is no guarantee of separation; anyone can bimble into it. And if that "anyone" is not transponding Mode C, ATC has no idea if he is in it, or below it.
All you get by flying in CAS is a reduced probability of hitting somebody. You can achieve a similar improvement by flying high; flying at 5000ft in Class G is far far better than at 2000ft in Class G. There is almost nobody above 3000ft, relatively speaking.
If somebody wrote
"won't be long before I get to read your autobiography entitled "AAIB bulliten 08/2007"
on my tombstone I would be very disappointed; I would have expected it to be spelt correctly
The only thing which has the
potential to protect against mid-airs is TCAS. This comes at a cost, £10k-25k, plus everybody else using Mode C, or (in the far future) ADS-B with broadcast of GPS position (which would be much more accurate and probably much cheaper to buy and install).