Those lights should be used exclusively for what they are for, fog and low visibility situations, honestly. Even in foggy situations, those lights are extremely bright.
Many people, here in the States won't know that you actually are blinding the ish out of them on purpose, using the lights when not really needed, but it's just common road courtesy(.-i.e. not hogging the left lane) to leave them off during times of good visibility. I drove cross country in Spain for a couple of years and have been the victim several times. Not cool.
Missed the boat on that one, eh? Aside from sending Morse code to your friends, which is admittedly annoying as hell, the rear foglight is very useful.
I use the foglight (singular) when it is foggy/snowy/rainy out, i.e., very low visibility. When I push the button, only the driver's side bulb illuminates, and I very rarely use it. It is bright, but not any more so than the brake lights. Read on for more on that. When I turn on my rear fog light, it is generally because I have noticed that I am having difficulty seeing the cars in front of me due to the conditions.
I also wired the bulbs on each side to come one when I hit the brakes. The foglight bulb is EXACTLY the same bulb as the brake light bulb - 21W, no difference in output compared to the brakes. So, it's just two extra bulbs illuminating when I step on the brakes, which I tend to believe makes people a little more aware of the fact that I am stopping (or at least slowing down).
What I like about the way I installed it is that, unlike on the new Volvo or Mercedes cars, turning on my front fog lights is independent of my rear fog light. It seems to be cool these days to not only convert your crappy headlights to poorly-aimed HIDs, but to run with all your fog/driving lights on all the time, regardless of conditions. Those cars then have that one irritating light shining out the ass end, in addition to the wash of light spraying out in front of them. I, for one, use them when I need them, and only when I need them.
I also have several years of driving in Europe under my belt - when the conditions warrant it, those rear fog lights are VERY effective.