Why someone would want this, is an excellent question that is more on subjectivity, than objectivity. Personally, I got it to support an alternative to mainstream consoles--Playstation, Xbox, Nintendo. I don't piicture anything like this becoming a big threat to them, but as Michael Patcher said, could be a nice second console. One that is just always hooked up to the TV, rather than having to leave the wire hooked up the TV. And as a practical matter, not all mobile devices have microHDMI and HML outputs. HML does not always give enough juice to charge a phone, but slows the discharge rate.
Some controllers need root access to the phone such as the Playstation Sixaxias, well the Sixaxias Controller application does. I would venture to guess that most mobile phone owners are NOT rooted. Some controllers don't such as the Wiimote and maybe some generic blue tooth controllers, but how well they work may be questionable. I know the OnLive universal controller works, but only well with OnLive; I've tried Dead Trigger and Shadowgun and wasn't able to program the buttons.
In addition, I think some people are hoping if there are enough numbers, it will attract more developers. This could mean more better quality games, and maybe some new interesting ideas that would otherwise be too expensive or risky on other platforms. That's part of the point of Ouya is for low entry cost into game development into the living room. I know that's part of my hope.
I figured it's only a $100. I've blow that without thinking with going out to eat or other similar activities, so it's not like I'll miss the money. It wouldn't be the first time I've gotten tech where the company went under. If were able to use Google Play or side load games, the console will be usable even if the company goes under.