Okay, so a few things. 1) Yes, this is far and above one of the most powerful Android devices on the market, that is by design. Many high quality games feature either manually scale-able graphics levels or will automatically scale up to provide the best experience your device is capable of. (See the GTA games, the Riptide GP games, the Shadowgun games, etc.) 2) This thing will run most any Android App for the forseeable future, can you say future proof? 3) This thing needs some serious horsepower to decode and re-encode the signals it receives and sends from your computer when doing PC Streaming. 4) This does have HDMI out, and can output at 1080p. It takes quite a bit of power to run 1080p.
So I guess the answer to your question is yes, it is overkill, but I look at it this way; This device will remain a competitive device on the market for at least 2 years, and will still be usable for many beyond that, and because it's Game Stream functionality allows you to play AAA PC games on your mobile device while offloading the majority of the work to a desktop PC elsewhere, it's gamestream functionality is essentially endless, assuming you continue to upgrade your desktop with NVIDIA cards. I'm going to buy a Shield because I could (theoretically) still be using it to stream PC games to my lap or my TV anywhere in my house 10 years down the road. This devices usability will last right up until the actual hardware fails, and from everything I've read NVIDIA put their heart and soul into this, and made one hell of a device.
So while many complain about a $300 price tag (and I agree, it's a lot, too much to pay maybe for a phone, or a small tablet) but you are getting a device that you can use for much longer.