ANSA - I have to disagree with you, and agree with Mr G!!
The amount of times a pilot in the LTMA has told me he/she has traffic I have called on TCAs, but actually mis-idented it is too huge a number to count.
I am calling traffic, I don't care if the pilot has it on TCAS - a simple 'Roger' will suffice... Similarly, the amount of pilots who say "Approaching FL... looking for further" then respond with "we have that on TCAS" when I tell them about traffic a mile away, 1000' above their cleared level is very annoying.
TCAS is not
an air traffic tool, it is a safety/separation tool. It can be useful in assisting pilots with some situational awareness, as long as the pilots understand it's limitations - especially in Azimuth.
There is a brilliant example in the NERC training department of a pilot trying to second guess ATC by using TCAS in the azimuth and adjusting heading slightly to miss an aircraft as he thought ATC were cutting it fine.
What he actually did was erode an estimated 15 miles separation into less than 5... resulting in causing huge confusion to the ATCO (an also a temporary suspension of their license until the incident was investigated)