Yes, let's step back into the real world:
1. Being held liable for guest's actions: http://www.huffingtonpost.com/2011/04/24/unsecured-wifi-child-pornography-innocent_n_852996.html
TL;DR - Man was accused of downloading child porn after a neighbor used his AP to do so. He was cleared after some time, but not after being raided by SWAT; arrested; and having his name and reputation smeared all over the media.
2. ISP TOS: http://ww2.cox.com/aboutus/policies.cox Part 1 "[...] You may not use the Service to: [...] Resell or redistribute the Service to any third party via any means including but not limited to wireless technology." Break the TOS and I break the contract, which is terms for Cox to drop my service. Get another ISP? Well, maybe you have a surfeit of broadband choices in SF, but in my neck of the woods, there's only two -- and the other has the exact same TOS clause.
Every bit of this comes back to my main point. There are risks and rewards associated with running an open AP. I've detailed the risks. Are the rewards worth taking those risks? For you -- yes. For me -- no.