It's use is verboten 'cos the CAA (when they feel like it) try to comply with ICAO regs, although it often takes a diktat from JAA/EASA to kick start it.
This is why we recently changed to hector pastilles from the 'nothing wrong except it's not ICAO' millibars, and why we have to use either 4 or 6 digits (not 5) when giving frequency changes.
After all, why say something as quick and easy as 'QSY' when you can require everyone to say something longer? What next I wonder, 'the sea level pressure setting is XXXX hector pascols' instead of 'the QNH is...'?