FC
Question, not criticism -
Take your point about what the Manual says, but, can't you identify aircraft on departure? to save the hassle?
Down sarf! as someone has previously mentioned, the majority of times we need to get an aircraft to ident are not for identification puposes but to trigger off computer tracking (not quite code callsign conversion but along the same lines) This sometimes fails to happen automatically if, say, an aircraft is on a direct track and some significant distance off route when entering UK airspace. Or it has arrived at the boundary very early or late. Unfortunately, as pilots, you'd never be aware of when this has happened until we ask you to ident - It's not that we can't see you it's that the computer can't!!!!