It's time wasting because if there is no verbal response the ATS operator might have to fully or partially repeat the instruction before the ident pulse is received. Bear in mind that the 'Squawk Ident' instruction could be relayed by another ATS operator on behalf of a radar controller as in a phone call "ask him to squawk ident". The readback is not unnecessary com otherwise it would not be a requirement in CAP413, the content of which is based on many years of effective RTF communication.
Similarly, the unsolicited use of 'ident' on departure is often wasted because the departure radar controller is not giving the departure undivided attention to observe the response and the action might have to be repeated at the radar controller's convenience.
'Squawk ident' should be on instruction only, and read back.