I suspect blanking half the hole does serve a purpose - either to accelerate the air or to keep crud out of the wheel arch!
Fog lights are pretty useless 99.9% of the time except to warn drivers coming towards you that you have cataracts and perceive fog where there is none (or that you are an idiot that prefers to drive with sidelights and fog lights rather than use your perfectly good headlights).
I'm in favour of connecting the fog light switch to a 30mph speed limiter (since driving when the visibility is poor enough to warrant the use of fog lights is dangerous). I wonder how many people would continue to use them :wink: