I've just bought a Nexus Player and I can't think of a really good reason to root it, but I bet there is at least a handful of which I haven't thought of. Sure, it's nice to block ads and perhaps, if you need space, hooking up a HDD. But unlike the Android OS what can you do with root that adds value to the AndroidTV experience?
I saw a post on Reddit with half a dozen of clever suggestions and bookmarked it, but unfortunately lost it recently when I restored a previous image of windows. Sadly, I can't find it now.
If you run a hosts style adblocked, like Adaway, on Android TV, does that block ads in YouTube?