OTC - Just to add my agreement with Cartman's answer to your question. I am a sim trainer and my predecessor used to teach the same thing, i.e. use the Pan or Mayday word with every transmission. So we had a big debate about it and the only reasonably valid thing he could come up with was that it prevented them forgetting to use the word on checking in to a new freq. which is a requirement.
So we don't teach that any more, but there are lots of guys around who were taught under the old system who, because (thankfully) their experience of using P or M is limited, revert to old practices under pressure.