More and more cars use daytime running lights. It helps me to see them. I definitely want them to see me.
As I am a bent rider and cannot, by law, get too nerdy, I light the thing up.
As you can see in my well-kept basement (we're scheduled to appear on Hoarders in about a month) I put a Bontrager light on the Brainbag, and two Topeak lights on the seat struts:
I installed a One Armed Bandit on the front, and I use a Mininewt 350 on the lowest setting, about 50 lumens. It lasts all day:
I feel a bit more secure out there.