A handful of the cable TV providers, including Time Warner Cable, are experimenting with letting Roku, the maker of a popular streaming Web-video device, take over as the primary access point for TV service. In principle, it means viewers can use one piece of TV equipment and one on-screen guide to seamlessly hop from an NFL game to “House of Cards” on the Netflix app to an Adele music video on YouTube. More cable companies should take this step of turning over the entertainment gateway duties to companies like Roku, or Apple, whose primary mission is making pleasant-to-use consumer electronics.