Reading back the QNH with the ATIS letter is the only check that LHR ATC have to ensure that you have copied/heard the ATIS QNH correctly.
In probably the busiest time in the cockpit, (emergencies aside), it could be rather simple to be distracted and mishear or write down the wrong QNH. By transmitting it in the manner that LHR require you to do, you have an independant cross check.
Independant cross checks are part and parcel of aviation. This one makes sense because as Gonzo states, it is the only time, if you state the correct QNH, that you will hear it at LHR.
It's not a difficult concept to grasp, it's not a time consuming thing to do, and it does have an added safety benefit. Maybe only one crew per year will get the QNH wrong, but if they can be corrected before getting airborne, then surely this check is worth it?
Lack of professionalism by the OP is what we should be stamping out!