TV in is an additional functionality.
You could plug in your PC, Apple tv, digital receiver or whatever you have to gain overlay functions.
The general benefit is that as many entertainment options as you you currently use are all theoretically able to be colacted in a single interface that you can control by voice, button or motion.
If you are a cable cutter, swap between youtube, hulu, netflix, crackle, hbo go and videogames on the fly. Whetever functionality you need for streaming to the device are either there or on the way. Cable card has mostly failed in the US so most people here with 300 million population use a STB of some kind. No need to act like having that capability is an oversight or omission.. in fact its quite the opposite.
HDMI is a global standard on CE devices so saying that allowing for HDMI in is waste seems illogical.Television is simply a tough nut to crack.. which is why no one has done it yet. This is not a bad start.