When Comcast rolled out the new cable modems in San Jose, they not only increased the bandwidth of the internet connection, they also removed the cap. At some point "real soon now", I expect some of the traffic on the Comcast side of the cable coming into my house will, theoretically, occasionally be used by someone driving by with the wifi enabled on their cell phone. I see enough "Xfinity WiFi Hotspots" on my own phone when driving around that I spect they're already starting this.
It seems to be a reasonable tradeoff for a considerably faster connection to the internet, and no longer having to worry about the bandwidth cap. (Which I never had gotten very close to, anyway, but it's nice that it's no longer an issue.
As for security, I set the Comcast's wifi up as my guest network, and everything I care about inside the house is firewalled off on the other side of my own router, running DD-WRT, with a different wifi password. I'm not any less secure than I was before.