The New York Times has an article about people "borrowing" WIFI from their neighbors. What it shows is that if you aren’t expecting to share your WIFI network, you ought to secure it. I say "expecting to share" because here in San Francisco, people do share their WIFI — but they also understand the risks associated with it. As this article shows, many people are sharing their WIFI without knowing they are broadcasting it to everyone around them and without understanding the consequences.
Also, this kind of problem impacts businesses as well. There are many small businesses who are using insecure WIFI within their offices. It’s pointless to pay for a firewall, if you end up broadcasting your network traffic to anyone with a WIFI antenna. And, don’t think WIFI is limited to the locations you can pick up a signal — someone with a directional WIFI antenna can pick up access points from *miles* away — 125 miles is the current record. And while they are using some modified gear, it is possible to pick up WIFI signals in San Francisco from the top of the Berkeley hills (about 20 miles) using fairly common hardware.