I love the Tesla fog lights, they have a nice wide pattern that illuminates the road periphery fairly well. It's also true that they seem to cause little or no annoyance to other drivers. I leave them turned on all the time (ie they activate with the headlights).There is probably no harm to leave the fog light option on all the time. On the road, I notice more than half of the Teslas actually have fog lights on when their headlight is on, regardless of the weather condition. The fog lights are not that glaring to my eyes - they are weaker than the LED headlights that are on almost all new cars now.
My car was delivered to me with fog light option turned on from the factory. I guess many owners didn't bother to change the setting and this is probably the reason why there are so many Teslas with fog lights on.
N B. beware of the current crop of aftermarket fogs with sequential turn-signalling functionality. The description of these often claim that they are brighter than the Tesla factory fog lights - I was very skeptical of this, and my skepticism was confirmed when a friend of mine installed them on his M3P. It's a decent amount of engineering that goes into the design of a well-controlled and well-distributed beam pattern; these aftermarket units may be better than nothing but they don't provide nearly the road-illumination Improvement that the factory units do. I would say you should buy them if you like the sequential turn-signal action and you don't really care about the lighting performance, or if you have an M3 Standard Range that doesn't have fog lights at all.
I have also noticed in the forums here, that there are a number of messages from the UK seeming to imply that the use of fog lights in non-foggy conditions is either illegal there, or at least very bad road etiquette. I was a little confused by this, but I believe they were talking about rear fog lamps, essentially those extra-bright auxiliary taillights that are not really common in the USA ( though perhaps they should be).