JD - the point HAS been debated ad infinitum.
The QNH has to be read back - along with other items - it is a mandatory read back.
It has become an item that is a mandatory read back due to incidents whereby this has been a causal factor.
There have been level busts caused by incorrect setting of QNH - I'm afraid that that is a fact. I'm sure if you contacted the CAA and requested data they could provide it for you.
During the entire history of aviation - procedures and phraseology have come about due to incidents, the history of which has been lost - but the lessons learnt are still in place.
You may well be fabulous and always get everything correct - but on a daily basis this is not the case.
louby