Invite-only creates hype around your product. Google wants people to Tweet and post on Facebook: "I got an invite into the new Google Maps!". Simple marketing.
I'm tired of Google request-an-invites, and it was a reaction completely independent of the comments here (which I hadn't yet seen).
Something to think about, I guess.
… on second thought, I'm sure even Google knows they'd be preaching to the choir there.