Comcast wants users to pay some form of RENTAL fee (cable box, CableCard, Roku app after Beta test ends) on fixed-location TV's. You will never be able to use Comcast TV without that rental fee.
If your TV supports it, you can see if the networks offer their streaming apps on your TV. If not, you are out of luck. And even if so, they may not have an agreement with Comcast to allow full access. For example, even with my Roku, Viacom and Turner networks can't really be used as Comcast is not a partner, so you cannot log in and get full access.