This is my favorite debate to troll normally but i'll pass today. I just like how people have such passionate opinions.
I have researched it, and the only thing that I can ever find that is backed up scientifically is the argument that more light is good for C3 flowering plants, making specific reference to no benefits to dark periods. On the same note, I think alot of people who see growth spurts from a dark period, they are probably not incorrect. In the dark the plant stretches to find the light, and will likely be noticably taller when you check on her again.
I've also seen arguments regarding plant processes that they can only do in the dark, however I've not been able to find anything to back up that idea at all except speculation.
On the same note, the energy saving from 18/6 could be reason enough. If you are vegging in a schedule where you have to wait for the flower room to clear, and have no trouble reaching desired size, then a schedule of less light can make the most sense.
Though I have seen the arguement that the same electrical usage is used for the same sized plant ultimately because they have to veg longer, thus use the same amount of electricty. I would disagree with this argument based on evidence I've seen, though I have not done side by side tests myself. I don't have a stable enough environment to test anything like that conclusively. My general argument is that at 18/6 light, I think plants grow somewhat slower, but not 20% slower, so it is argueably more efficient use of the light in that respect.
That being said, I try to get as much as I can out of every minute of veg and personally run 24/0 exclusively now.