I would argue that there is very little value in off tv play due to the low quality of the touch screen itself. The poor image quality and resistant touch screen detracts from the experience. I don't look at the screen because it is terrible to look at. I don't interact with the screen (or as little as possible) because it is terrible to interact with. I sold pikmin 3 because I couldn't stand having to use the touch screen so regularly with the map. It ruined the game for me.
As much as people like to pay out Apple with their ideas and opinions, they were 100% right when they said that making a product beautiful and easy to use makes people want to use it more. The same can not be said about the gamepad.
Lower quality is better than not being able to play at all. Higher quality can actually be distracting. When I play MarioKart on the TV in single player mode, it looks really nice. That's actually distracting. I shouldn't be noticing the visuals, I should be focusing on the race. Also, it can be more comfortable to lie down on the couch using the gamepad, rather than sit up and face the TV. I find myself looking at the gamepad all the time, wondering why I am wasting electricity leaving the 50" TV on. Plus, if you are playing a game with text input like Wheel of Fortune / Jeopardy, I was happy when I figured out you could just touch the letters rather than scroll around the letters all the time.
I don't need my Wii U to play MKV files. My TV can play MKV files directly from USB, but what I do is use DLNA to stream it straight to my TV off my laptop. Of course, WIFI is only fast enough to stream 720p well, but that's good for most TV shows. When I want 1080p, I just plug the laptop in with an HDMI cable.