Ok, "we all" know that a good skulk is a harassing skulk: Keep the marines busy.
However, there is another way of looking at it too: When you're a skulk, you don't receive any resources. The resources you DON'T get, will go to the rest of the team, mainly the gorges. That is a new hive, faster.
So, i will introduce the new mantra you all should be repeating: The only good skulk is a dead skulk.
(Ironically, the marines may agree on this ..)