The reason for the readback is principally to mitigate the hazard of transcription errors - 979 instead of 997 or, in the example Sky Wave offered, 1002 instead of 1022. Differences of 1 or 2 hPa really are of little operational significance to ATC nor, I am told by pilots, to the operation of the aircraft.
The UK-specific procedures may be seen as outdated or whatever but they are all there for a reason. Having seen a transcription error of some 20hPa made in the met ob, it was interesting that two aircraft departed without comment before an inbound pointed out that there was something very wrong. There has to be a balance between belt and braces and practicality but I would suggest that there are less critical procedures that could be dispensed with than the QNH readback which might be the only cross-check with a human before the level bust!