Far better that a responsible regular passenger has some idea of how to get the thing on the ground if the donkey in the left hand seat fails, whether they are shown the basics by a PPL or rather more formally by an instructor.
We probably all know of stories where a passenger has been talked down over the radio; off hand I can think of at least two in the UK and one in the US in the last two or three decades. Doubtless there are crash reports to be read where a PPL has expired in the air leaving his passengers to an awful eventual fate. The limited experience I have been given has left me with a reasonable and, I think, justifiable confidence that I could certainly get most light aircraft back onto a runway and probably in one piece too. (An airliner? Well, I'd give it a go...
Talk Down by Brian Lecomber provides as good a reason as any why The Farmer should pursue this idea - and it's a terrific read as well!