Tough Crowd...
Nearly 300 views and no bites. I'm not really sure what to make of that.
Okay: I'll go first. It's primary altimeter QNH - always was, always will be; for any number of reasons, including the failure cases.
Why I asked the question is that my new airline has recently adopted a procedure at transition where the primary altimeters and ISIS are simultaneously set to QNH extracted from the FMGS Perf App page. Bear in mind that we operate exclusively under radar control, and most if not all destinations provide METAR/ATIS by ACARS or Voice. And, we've been given QNH when cleared to first altitude below transition. And, we could always ask if we're not sure.
So, I wanted to confirm my belief that QNH on the Perf App page was like a kiss from your sister - nice, but ultimately useless.
Essentially, my new lot are using the App page as a scratchpad - to which I say, why not use the QNH you've just received or written down as basis for the crosscheck, and cut out all that extra button pushing. That's a good reason, right there.
But the problem is deeper than that: we're now using an input to error-check another input.
I gather this new procedure is in response to a small number of mis-sets under workload; but I really think that the procedure makes things just that little more complex, doesn't trap the error correctly, and has no basis in system design, anyways.
I leave the floor open...