Eh, it's not a bad idea. I'll buy the occasional guide, mostly because I like seeing everything a game has to offer, and I no longer have the time to play 300 hours to find everything. So it's usually for things like GTA and Final Fantasy that I get a guide.
And sure, gamefaqs may make guides irrelevant, but it takes a few weeks for the good guides to start popping up on gamefaqs. The authors need time to write them after all. So if you want some help right after a game's come out, a guide's your friend.
That said, I'm not sure what benefit this really offers over having a hard copy. (except that they're currently on sale.)
Still, it's a way in which Steam is trying to improve itself, which is laudable.
Creston