Bruce Schneier, author of Applied Cryptography and other books and articles on security and privacy, leaves his wireless network unprotected. My first reaction was probably the same as yours. What an idiot! He then goes on to make a few points that made me regret my first reaction. Bruce highlights the many other ways we take security for granted including leaving doors unlocked, drive in the rain while on a cellphone (whew!) and make other security sacrifices for the sake of convenience.
He explains that while it leaves your computers more vulnerable to leave your wifi open, you are exposed when you use your laptop in a public location anyway so you had better learn to secure your computer.
While I agree with nearly all of Bruce’s comments, I think i’ll keep my wifi secure. Oh yeah…i’m going to start locking the back door when I leave the house now as well! 
