I am also a Comcast customer. The cable connection is through an old, weak cable that goes through the apartment downstairs, and it slows down my connection a bit, but that is tolerable. To fix it, they would have to rip apart the walls in a bedroom occupied by an eight-year-old girl, and I don't want to put any child through that trauma. If I allow Comcast to share my cable connection, then I might be slowed down to an unacceptable level.
Also, their new cable modems DO NOT come with a battery backup -- they make you buy the battery from them.
They say that nobody can take advantage of you without your permission. Well, I'm paying enough in cable bills, and I'm not going to let them. Unfortunately, FiOS is not available in my apartment complex, so Comcast has a monopoly.