A/C to A/C comms are useful in some situations, for instance if an instructor and student are heading out to x training area and i'm on a solo bimble they may not want me in the same airspace so we can and do nominate east or west when departing:
ABC you going into the X sector
Yep sure are CDE
ABC i'll go west then
Rodger
or in the circuit we communicate informally too, say if a student is practising a glide approach or somebody is doing something funky we normally let the others know. Especially if one of the Ag tractors is doing some fast trun arounds we may communicate individually to maintain SA and separation.
Doesn't stop the muppets though who can't oreintate their approach plates to north.... or glider pilots.... sorry chaps we know we have to give way to you but flying a directly OPPOSING downwind isn't really fair.