The PFA did research into all the mid air collisions over recent years. The results ;
a) there were very few
b) Mode S would have made no difference. Interestingly, most of the aircraft involved had mode c.
Rod1
Could you post a link. While I would agree mid airs are very infrequent, and given that fast jets, gliders and GA (the main players in mid airs) don't normally have ACAS - I can see that transponders wouldn't make a difference in these situations. However, I am surprised by the Mode C comment in that a number of the cases I remember involved gliders, microlights, and other aircraft that had a high probably of not having/not using mode C.
Also, as the price of TPAS and ACAS systems comes down this will be less and less true.
The most valid argument for transponders in my mind is the worry that one of the dozen or so very near misses a year between CAT and infringing GA traffic (generally not Mode C/S) becomes a hit.