Actually there are multiple reason for the day light only rule (it is actually only a strong suggestion, The Network Q runs into dusk, and the Monte has two stages at night.)
Some reasons for it;
One is very much from a TV point of view. Sorry guys but lets be honest, day light look a hell of a lot better on TV and...
Two, the lights from the cameras on the corners don't blind the drivers. It is hard for us to know exactly how a car can be affected by our lights, we try very hard to never shine our lights directly up stage. So the camera images you get at night, basically suck. Ask someone like Karl Scheible if we should run night stages with cameras.
Third, Worker Safety. Again, it is twice as easy to get hurt in the dark.
Fourth, Crew Safety, Service in the light is much easier and less of a 'Keystones Cops' routine in the light.
Fifth, booze. Spectators are more inclined to drink at night then in the day light. Less booze, means safer spectators driving roads like the rally people they just watched.
Sleep, retail businesses are open at the end, getting crews out, delay coverage, etc etc.
So, if someone asked me if we should adopt a day light rule in the US. I would respond "for the commerical viability of the sport, it makes sense for the powers-that-be to bring this rule in. Do I like it? That doesn't matter if it means my crew, the spectators, and our sponsors will be safer and happier."
Soul impact did some work for us (speed channel) at OT, at the end of his video is a good example of what we can shot at night.
http://www.soulimpact.tv/OT_ss5_6.mov