Its still very much whats the point.
It's not going to replace PCs or appeal to everyone, that's for sure. Still, if you can't always take over the big TV in the house this would at least give you the option of using it without having to move your rig. If it streams media off your main rig as well as games it could also save you having to set up a laptop any time you want to watch a movie or put music on.
It's obviously not a PC replacement, it's a media centre box with games. It's not like media centres are a new concept, there's already a market for it.
As well as few pc games being good for a controller.
It's not that hard to use wireless mice and keyboards. Besides, the whole controller vs M&KB thing is really personal preference: a few friends of mine have moved to Steam from the Xbox but they prefer to use a controller when at all possible because they're used to L4D2 and so on with controllers on the Xbox.
Whether or not this whole SteamOS/box thing floats depends entirely on how Valve can tie it together. Nobody is going to replace their Windows box with it so they've got to convince people that a media centre which streams games is a good thing. That's going to massively depend on what this media deal they've hinted at is and what else it can take off your main PC.