If you're a bike ninja and no one can see you, your only defense to not getting hit is to being able to see everyone else. Thus you "need" to ride against traffic.

This spring I accidentally ended up needing to bike across town after dark, and I didn't have a light. It was pretty fun (though the vast majority of my route was on bike path, not streets). It's really amazing how much you can see when you don't have direct street lights on your path but you do have ambient city light. I was really surprised to find I didn't really need a light as long as I was the only one on the path. There's was like 1% of the path that went under a bridge where I couldn't see, but that was about it.

Ironically, the only real danger to me under those circumstances were *other* bike ninjas! lol They were moving fast and since neither of us had a light we couldn't see each other until it was to late to be able to stop. I thought it would pretty ironic if we crashed...