I can think of several situations where an air-to-air frequency would be useful, but they all involve flying in very inhospitable terrain or out of ground station range over the open sea, (ie mostly where 123.45 can legitimately be used.)
It might be interesting for the CAA to "create" a spare frequency for this, as a trial for a few months. My guess is that it would degenerate rapidly into a CB chat channel and be withdrawn.
Gliders and balloons in the UK have such frequencies, and from the little I've listened on them are reasonably disciplined - but they have a specific and important purpose.