Setting aside 'expert opinion' and legality, and being 'practical' for a moment, I do ponder whether the rest of the real world is so anal about such matters.
Assuming you have looked at the synoptic chart for the day (knowing how to read it helps!) and have a valid forecast with area QNH and TAF QNH, you should have a reasonable idea of QNH. By whatever means (phone, radio, PIDOOMA) you dial in a QNH for landing. It turns out to be 3mb in error. If it is more than 3mb, maybe you need to go back to school and study meteorology. Or you are flying a Global Jet to the far side of the world.
Even compounded with an allowable 60ft aircraft altimeter error, worst case could have you 150ft low on an approach. Are there any non-precision approaches out there where such an error would cause impact with obstacles? Granted, it would not be a pretty sight on a Cat I ILS to be 150ft low at DH, but it is highly unlikely that you would ever encounter a non-approved QNH source at an ILS-equipped airport anyway. And if you didn't do your glideslope altimeter check you were a naughty boy. Ditto on GNSS approaches - there are plenty of opportunities to correct for altimeter error before you get too close to the dirt.
Plus, there is always the option to add 100ft to your minima if you are uncertain about the accuracy of QNH. Then you could still be 50ft low, but if you crash in that situation, it certainly wasn't the QNH that was your undoing.
Just don't have this conversation with an examiner on an I.R. renewal, because we all know how they love their approved QNH sources.
Last edited by Mach E Avelli; 4th Jul 2013 at 23:22.