Actually, as far as we are told, this started out as a request of FRA-Airport-controllers. Apparantly it is more often the wrong crew to return the correct clearance than the right crew to return the wrong clearance. Therefore, the controllers want to hear the callsign first. This, just to know if they should listen to the rest of the clearance, or start preparing to correct the mistakes in time.
On an additional note, answering with the callsign first, then repeat the clearance, forces me to memorize the clearance more conscious.
Best regards,
Var.