I was hosting a server, and me and a few people were inside of a house that was pitch black. I have max shaders on, so I can't see stuff inside the house. The person next to me, on the other hand, suddenly dives into the darkness and navigates it with no problem. I had to use my little light thing to find my way through, whilst this guy is magically guided through the darkness like there's lines telling him where to go. Actually, that's exactly what it is.
Extremely bad for servers that are attempting, say, stealthy things that use darkness to it's advantage.