With temps pushing 60F in New York today, I started thinking about getting the bike out of the basement to squeeze in a few winter commutes. But commuting is a psychological thing for me. I need to be either all-in or all-out. I can't ride one day, drive three weeks, then ride one day. My head has to be "right" for commuting and my bike has to be all set up (bags packed, lights & mp3 player charged) so I can just get right out in the morning, before I have a chance to talk myself out of it. If there's an extra minute to feel the slightest chill and change my mind, I'm going to end up driving.
For me, the only fair weather commuting is during fair weather season.