I've recently started a casual job, working as a relief van driver. When I went out to learn the job with one of the 'regulars', he instructed me in the procedures for fuelling the vans. This included the instruction to "go to whichever diesel pump you can get to quickest. If it happens to be 'Optimax Diesel' just chuck that in; the company will still pay for it".
So perhaps many of the purchases of 'premium' fuels are made by people who aren't actually paying for the fuel?
Incidentally, I happened to see a repeat of 'Fifth Gear' on TV yesterday, where they tested 'premium' fuels. There were significant improvements in the brake horse power of a high-performance car (Subaru Impreza) when using those fuels, but no difference whatsoever when the fuels were tried in a normal family saloon car.
Chris