separation
Ferris,
Methinks you are being a little pedantic. If I have two a/c in proximity to each other and they are on their own nav, they can turn unexpectedly and come into confliction. If they are maintaining level separation, then this is not a factor. 'Track direct' is not a recognised technique, nor does it ensure separation, however nice the idea may be.
If I want to do a climb through, I will instruct them to fly or continue a present heading as a radar heading thus locking them on to a 'virutally' non deviating track.
We are not stupid, we know winds change, what we are doing is locking a/c onto a resultant track. Any Atco that did this to ensure the minimum 3 miles separation would either be very skilfull, or very silly. The A/c in question may pass each other with ten miles separation in the end, but if they are crossing tracks at oblique angles, a 15 degree heading change by one or the other may make a comfortable situation become sticky. Hence the locking of headings. It also means that a busy ATCO can get on with the other controlling tasks without constantly monitoring the climb through.
If, whilst locking A/c onto headings we can give one a direct routeing to a waypoint, then it's a win win situation for us and the pilot, who gets more direct routeing and an earlier climb/descent to final level.
Working in the London TMA, we have to use headings to achieve what we need to do. Whether these are controller chosen headings or 'locked on' pilot headings is a moot point if it is providing the required separation.