Sandman, unfortunately the correct readback of the QNH is something that we need to do, least not because pilots don't always have the latest ATIS and the QNH could have changed since. Also, it's the law!
Besides, clearance delivery (or Ground Movement Planner as we are offficially titled) is usually the quieter/less complicated position, where aircraft are not moving/liable to hit each other, etc. and we can happily repeat instructions, requests etc over and over and over again if required.
At Heathrow, when the QNH changes, we broadcast it on all frequencies without the need for readback. It's only in delivery that we check you have recieved the QNH, when issuing clearances/start up, as you may not be monitoring the frequency at the time, whereas on the other 4 frequencies we expect you to be listening out. Indeed, there is a box on our "flight data entry" (electronic flight strip) which serves the purpose of recording the QNH you have reported to us.
I know 1mb is about 30 feet, but what if the QNH was copied wrongly (heaven forfend) from the ATIS? or there had been 2 changes (could happen) since you recieved the ATIS. And let's not forget how tightly packed aircraft are, especially in the skies above our fair capital city (and my house) - we don't really want less than 1000 feet vertical separation and TCAS etc going off (sorry for the tabloid style sensationlism)