Look, even I[/i] run around rural areas looking for wifi signals so I can check my email.
Now, had I happened to connect to your network, and had I suddenly seen all of your workgroup folders, one of which contained that little file that had your credit card number written down, what would happen?
(Not that I am able, mentally, to steal such a number and abuse it or anything, but, point still exists)
Many people just run around searching for signals. Some of those people have bad intentions. Secure it!