I'd say that a single foglight on the offside is now *more* common than it used to be. I'm not sure whether it's penny-pinching - fit as few reversing and fog lights as you can - or whether it's a conscious decision not to pair the reversing and fog lights, thus preventing a) both sides of the road behind you being illuminated when reversing, and b) the fog lights acting as high-power tail lights, defining the width of the car to vehicles coming up behind you.
With the exception of a my first car (a Mark 1 Renault 5) and my present Peugeot 308, all my cars (VW Golf Marks 2 and 3, Peugeot 306 x2) have had dual foglights and dual reversing lights in the cluster, though in each case, the car was supplied with only a bulb in the offside fog light. My present Pug 308 (and the Renault 5) only have one reversing light (in the nearside cluster) and one foglight (in the offside cluster) so I can't just add the missing bulb to restore paired fog lights. When I'm reversing at nigth, I tend to put on the fog light to illuminate the right-hand side of my field of view so I can see *both* gateways that I'm reversing between.
I wonder if there are some countries (maybe France for the Peugeots and German for the VWs) where it is actually *illegal* to have paired fog lights, which is why a bulb is only fitted in the offside cluster of those cars which have sockets in both clusters.
My feeling is that *all* lights, with the exception of the central high-level brake light, should be paired.