I agree with Dook. As an ex display director anything other than type callsigns would have been very confusing. When Lightning/Jaguar/Tornado etc called 'running in' at least you immediately knew what was going to emerge from behind the chimneys/over the hill/out of the murk without having to hurriedly convert an alphanumeric callsign from one's notes.Moreover the other a/c on frequency knew too..