I ride year round - rain, light snow, hail, cold, hot, thunder storms - I'm in the dark a lot. Commute is 22 miles a day.
I used to use 1 tail light but got tired of seeing cars riding up on me in my mirror. I built a contraption that could hold 3 lights and believe me, when you have a lot of lights the cars pick you up quick and start moving over a lot earlier.

My last contraption to hold those lights wasn't too sturdy so I started thinking about it and remembered that I had a Topeak bar extender I wasn't using. I took it apart and rigged it onto my rear rack and works great. Ugly as hell but works great.

http://gallery.me.com/landsw