You will never get a straight answer on this question. I asked and the thread was deleted the next day. Riding past dark in parks is prohibited everywhere. Now say you were out riding and got lost and couldn't make it back out of the park before dark...and lets say you had lights...it would be in the interest of your safety and other lost riders safety to use them. Sometimes this happens with group rides, they fall behind schedule and end up in the dark, or so I hear.
A small yet hardcore fringe element of off road shadow puppet fighters exists however little is known of them and their violent illicit activities.
People speak of road biking as a dark side, 29ers are another dark side...but there is only one true darkside...that of the night.