gasax / foxmoth. I aggree with what you are saying, but the way I see it is, most a/c today have a radio of some sort, airfield info is obtained from outside the circuit, so what is the point of flying overhead for a look?
One scenario: East West runway, elevation 400ft, G-AB departing the circuit (left hand) to the North through the overhead (common at the field I have in mind) at 2500ft QNH, G-CD joining overhead from the North at 2000ft QFE, also common & regular. Does this not produce a separation of just 100ft? +/- numerous student pilot / low hrs inaccuracies.
Also the departing a/c may still be climbing from the (high downwind) point towards or through the point of separation.
Which brings me neatly to the issue of the QNH / QFE debate!!