Blimey, no wonder this thread is 4 pages long.
because not all pilots get it right when listening to the ATIS, the ATIS is not always right and not all pilots are as dilligent as you checking the elevation against altimeter when lining up on the runway.
There have been countless incidents caused by aircraft departing EGLL on the wrong pressure setting and I've corrected QNH readbacks more times than I care to remember.