Even if you don't mind strangers eating your bandwidth and doing questionable/illegal things through your ISP connection, I would think that in most cases your standard wi-fi router is the first line of defense for your home network. By leaving this open, you are inviting potential attacks from behind the WAN firewall, relying on your PC malware defenses to protect you. Doesn't sound like a good idea to me.

WPA encrypted and MAC address filtered here! Go to a Starbucks, ya freeloaders!