A couple of my own experiences post NAS:
Experience 1:
Provided radar advisory to an identified VFR in G (yes, it's overservicing, but he was correctly asking for a clearance into class C at the time) about an unidentified track 3 miles at one oclock, roughly opposite direction but crossing at the same level. Pilot response - "looking" (good).
At one mile apart, offered the added info that the miss would be less than half a mile on the left in about 20 seconds. Pilot response "still looking (good) followed immedietely by "traffic sighted, avoiding" (thank Christ) and observed a quite sharp turn of about 40 degrees to the right. The unidentified target didn't utter a peep or change course by one iota....was probably listening to a nearby CTAF, MBZ or John Laws for all I know.
In this case I am certain that the unidentified aircraft did not see the conflicting traffic.
Experience 2:
Same airspace, an IFR aircraft being identified in class G prior to clearance, offered radar advisory about an oposite direction unidentified aircraft at the same level Sighted when abeam at less than a mile. No radio contact.
I cannot be sure whether the unidentified aircraft did or did not see the conflicting traffic.
Before this NAS frequency rubbish both of the target aircraft would have been on the FIA frequency and probably (in my experience) heard some of the reports of the other aircraft or our communication about them, and piped up.
Regardless, in both cases my choices as an ATC were to believe in the new system and keep mum, or to protect the flying public as I see fit. To put it on a more personal level, I would hate to have to defend my actions in a court of law if any of them had hit and I had done nothing.
The point? See and avoid doesn't work in isolation, but as a part of an inclusive system (particularly including pilots listening out on the appropriate frequency) it used to work.
Thanks Dick.