Because sitting on your couch/lazy boy with a controller is the best way to play games, especially since your friends and family can join you for even more fun. It's better to have to option of using a mouse for RTS games though.
I'm thinking that the only thing the Steam Box will do is redefine a new minimum standard for games... if it "runs on steambox" then you'll automatically know it's worth buying (for those people who know nothing about computers). That way they don't get home and find out "what? minimum requirements are a 660GT and 8 GB of RAM?" and they only have an integrated card and 2 GB.... or whatever.
I don't think it's going to change PC gaming more than that (so far anyway). It depends on what these last two announcements are...
It's not like Valve's games take advantage of PC in any way that isn't mods. They're all pretty slow paced cinematic affairs.
When has this ever been true? If you want to talk about areas where Valve sucks you can say that Half-Life is just a bunch of boring arenas strung together with bland vehicle segments and a poor story. Portal is just a bunch of puzzles only worth a single play through. Then you get the multiplayer games which are as far away from 'slow paced cinematic affairs' as you can get.