I have to disagree. Bandwidth is a fixed quantity for which the owner of the AP pays. Using unused bandwidth without permission is harmless enough but rather rude; competing with the legitimate owner for bandwidth is theft.
Secure your router and it shouldn't be much of an issue. There isn't any perfect security but intention to secure is demonstrated even by the presence of a WEP key. Leave it wide open and don't expect a lot of sympathy if freeloaders make your activity lights dance.
Free access isn't necessarily benign, either. Some idiot threatens to assassinate the President over your router and whose IP addy comes popping up on the Secret Service list? There's a way to prove you're innocent if you're logging the right stuff and know where to look for it but how many private owners bother?
So theoretically, I could listen in on my neighbor’s wireless phone calls if I could figure out how to do it. In the old days, scanners could pick them up.
Not only that, but the router may only support a small number of different IP addresses and you may be denying the rightful owner of the router from attaching all the devices that he/she wants to at any time.