I have been biking to work for 6 years now (we just have 1 vehicle for the family, a mini-van for my wife). I do it to save money (i.e. gas/insurance, etc) and keep healthy. I do not ride my bike out of any desire to save the environment or help national security by reducing a dependency on foreign oil.
One of these days I'll put a sticker on my bike saying "my other car is an SUV" just to get all the tree-huggers off my back assuming we are kindred spirits when they see me cycling.