Comcast uses separate network antennae's one for your private use and the other for anyone with a Comcast connection. While I like to believe this is only meant for guest use like Comcast explains, I have a hard time not thinking a creeper down parked in his car close to my house will also be able to use my signal for who knows what, as well as possible slowing down my bandwidth. While Comcast might think this is a great feature, I know that opening up your network to any unwanted usage is never a good thing. Why risk the exposure. People once said Apple was virus free. Like anything else, someone will find a weakness in the system exposing you home networks to attacks.
So the question is how do you turn this feature off? You would think you could go into the router\gateway and just turn off the option in the settings. Nope, you have to call Comcast and request them to turn it off. While you probably wont get attacked or hacked, why bother. You can setup your own guest feature which Comcast claims this does. I would never have this option enabled. Take control of your own network and call Comcast and turn it off.