1
"All your base are belong to us."
Let's face it: there are lots of geeks out there--geeks with spare time whose idea of fun is to find unsecured wireless networks, hack into them, and leave dorky messages such as, "Ha, Ha!" "FREE RAFA--HACK IS NOT A CRIME," and "Best regards to the world." And they're the nice ones.
2
You paid for that bandwidth...
A little charity goes a long way, but when you notice that your CNET Hot Houses video plays choppily or that your podcast has become old news during the time it took to download, you may have one or two wireless parasites too many.
3
Don't let the terrorists win.
Security is big business these days, and snuggling into a WEP/WPA blanket lends a certain amount of comfort. Setting up your router's security protocols may not thwart Al Qaeda, but it could help you sleep better at night.