WWW, I think you have missed the point.
I agree that 121.5 DOES get over used and I am certainly NOT suggesting that every student pilot should be routinely calling D&D for training fixes.
However, the amount of air time occupied by a properly executed training fix call and reply is usually a lot shorter than a PAN call and the subsequent run of questions and answers, which can really clog up the frequency for some time. In my opinion, if a student becomes concerned about his navigation / position, it is surely better to call for assistance early rather than leave it until he is definitely completely lost and he has continued into regulated airspace, a parachute zone or a danger area. Prevention is better than cure, a stitch in time saves nine, etc. A “training fix” consists of a quick two way exchange, all done and the frequency cleared asap. Student has now confirmed his position, safely on his way, end of story. This is nothing new BTW, again, it IS in the CAP!
On the other hand, a lost PPL blundering about, not talking to anyone whilst possibly looking inside at a "£100 GPS" which he can't interpret properly causes chaos on more frequencies than just 121.5 - ask Luton Radar, or Brize Zone, who seem to have suffered more than their fair share of airspace busts this summer.
If a pilot deselects or turns down the volume of guard frequency to an inaudible level and doesn't turn it back up again, it is a CRM and cockpit procedural issue.
The most common “nuisance” calls heard on 121.5 (almost every day in UK over the last few years) seem to originate from commercial aircraft. Some pilots on scheduled IFR flights, having misdialled their next frequency and not having the sense to go back to the original one result in London Centre or other ATC agencies being obliged to attempt contact on "Guard".
I wonder what is going on in those cockpits but it may well be because the NHP never wrote down his previous frequency and can't remember it once he has twirled the dials. That’s a CRM and training issue coupled with poor airmanship.
Another source of misuse of the frequency is when the wrong transmitter box is selected. On 121.5 we hear the initial call, no reply, repeat of the initial call, followed by someone replying "You're on Guard". This can be repeated a couple of times where the same errant pilot has turned down his 121.5 receiver volume and can't hear any reply. This mistake is often made by commercial crews (some airlines seem to be heard more often than others, but that's a separate argument). I hold my hand up (and my head down in shame) and admit I have done this too, but only once of course (due to unfamiliarity with a transmitter head; we weren't using our regular aircraft). I apologise to all of you who listened and possibly turned down your box 2 volume in disgust. I hope you remembered to turn the volume back up again.
A hapless GA pilot in a remote area outside range of D&D? Is there evidence of this being a real problem rather than supposition? Where are these remote areas of UK? A hapless GA pilot in a remote area with a problem, i.e. uncertain of his position is highly likely to turn UP the volume of 121.5, not turn it down, or at least should do if he has been trained properly.
Fighters being scrambled against airliners? Once or twice in recent times in UK, as a result of an increase in the nation's security state. Exciting stuff, but in the big picture this has happened very infrequently indeed and cannot seriously be blamed on a student pilot calling for a training fix!
Without wanting to be disrespecful, and without using bad language to you, especially as you are now a moderator
it appears that you have jumped on a pedestal over what you see as a student "honesty" issue. You are the only instructor I have ever heard of to routinely get on the phone to try and "trap" his students away on solo navexes. Did your RAF QFI do that to you? I never did, it certainly wasn’t the normal thing to do during my time instructing on a UAS (admittedly some time before you became a member of UWAS, so it might have changed). Don't you trust your students to debrief you honestly? I always did.
Good discussion but best to listening to the other side of the argument without getting angry.