Aside from the practicality and price reasons I already mentioned, another reason it's silly to say that the Ouya offers nothing over a cheap computer is that the same could be said for ANY console. Why do people buy consoles at all when a $100 processor and $50 graphics card lets you play Assassin's Creed, Darksiders, Skyrim, Gears of War, etc. on your computer, and a computer does so much more on top of that?
And honestly even that's moot. Again, why does the PS3 even exist? What does it offer over the Xbox 360?