If you argue that something is useless or inferior to an alternative, then you need to prove that it is that for everybody, not just for some people.
No I don't. I don't have to prove anything in this discussion, because I'm posting on an Internet message board and voicing my opinion. When I'm trying to get published in scientific journals, or if I'm on trial for something, those are the times I need to prove something.
And in this case, I'm not even arguing that nobody should buy an XBox. I don't care what people do, and if the XBox is, for whatever reason, more convenient, by all means go buy one. I don't care very much what some random asshole on the Internet buys. You could go buy yourself an XBox for every room of the house, and hey, it's your money, you can do that. I'm still aloud to post on the Internet that it seems excessive and stupid.
As far as whether streaming PC games to your XBox is a good idea, my guess is that the experience won't be great for twitch games, but if it's a good enough experience for you, then again, by all means, have at it. I'm not going to be an asshole and claim that you have to prove that's the best choice for everybody.
At the same time, if what you really want is a method to stream your Steam games to your TV over your home network through a set-top box, I would guess that there are cheaper solutions than the XBox. You don't even need something with much 3D rendering performance to stream video.