Just a guess but I would imagine that the readback makes sure the controller knows that it is the correct aircraft that is Identing and not someone who has misheard the instruction.
For example if G-ABCD starts talking but G-WXYZ comes on frequency & interrupts it is entirely possible that G-CD could be asked to Ident but as G-YZ didnt realise G-CD is talking they might wrongly interpret the instruction as being for them and Ident. If theres no readback ATC would then accidentally believe that the RADAR trace for G-YZ is G-CD
Last edited by Burnie5204; 23rd September 2012 at 17:34.