Blog v/s Intranet

I've been trying to get the advertising agency I work for to start up a group blog. Unfortunately I can't think of too many good reasons, apart from other cool agencies have blogs, to start one. Can you? Wieden & Kennedy has a blog. Fallon has a blog. Ogilvy has a blog or four. A few other good agencies have blogs. All that said, what's the big idea behind a large agency starting up a blog? Think about it before you go around trying to champion a blog for your agency. And then, tell me. Personally, when it comes to a large agency, I believe an intranet is a better idea.