I always like to play in the dark, but there are some reasons not to.
Playing in the dark, if I could see myself from a 3rd person, makes me look like a freak. I always harp on my friend who likes to play in the dark about doing this, since it seems so depressing.
Another reason is that its kinda hard to see the keyboard (finding the # key was a little hard for me in the dark as a n00b).