Originally Posted by mushroom69
what possible usefulness is the read-back?
For a start the ATCO will know that the addressed pilot has heard the instruction and will action it. No different from reading back a heading which is a turn for identification.
Non-readback (and unsolicited idents for that matter...
) is of no use at all. Non-readback is also laziness, time wasting and discourteous. If the correct phraseology cannot be remembered (Wilco) I'd rather have a "coming down" or a "flash" or a "there you go" or a "Howzat" than nothing at all.