Heard another (supposedly) official rationale the other night (via another driver).
Apparently it's for the benefit of the emergency services
So if they roll up and there's a fire, the sticker helps them find the fire extinguisher (so why do the rules say "if the extinguisher is hidden from view the position must be clearly marked in view of the driver and
passengers 
)
Well presumably it's not for the benefit of the firies, because, you know, presumably they have their own more specialist equipment for dealing with, er, fires...
So maybe it's for police and paramedics - maybe it's just me, but I'd have assumed they'd have their own stuff for dealing with fires, and stuff that's more accessible to them than fannying around looking for stickers in the taxis and then finding the actual extinguisher, which will generally be in the boot.
But which all seems to me like an even more contrived rationale than the first one about it being for the passengers' benefit if the driver lost consciousness.
But I'd guess the more likely form of unconsciousness would be that of passengers because, of, er, stuff, and most likely young females.
And I wonder which group of people in positions of power should maybe except some of the blame for that?