Shy, you don't say "PAN PAN" across the cockpit for the very obvious reason that if you did have your finger on the tit as is all too possible you'd have just broadcast a real one, just as we used to say in the military before setting off practice emergencies, "For Exercise, ......".
During my fifteen years or so as an RAF QHI and QFI I never had one student press the transmit switch by mistake in those circumstances.
Students tend to do what they have practiced. If they are only ever being taught to say "Practice Pan" without transmitting across the cockpit, chances are they would latterly say "Practice Pan" under real emergency conditions, when the pressure really exists.
Does the military still have 243.8 as an alternative to 243.0? It does seem logical to have an alternative VHF frequency for training purposes. However, as I've said before, if my concentration gets spoiled by chatter on 121.5, I just turn it off on the select switch then back on again after a minute or so, to check if it's quiet again.
So you turn guard down, or off just so you can concentrate on ATC. Why don't we simply turn it back on after a few mins? Well, because there is a psychological aspect to PLOC, and we don't always remember or notice that we have turned guard off.
For the forgetful ones among us, there are ways of reminding ourselves that something needs our attention.