You can already buy Office or CS online and download it. My local electronics store even sells cheap Office licenses with nothing but the key. Microsoft or Adobe don’t need the App Store.
The Mac App Store isn't the game changer some people seem to think (or fear) it is. It could very well be a boon to small developers, or it could have no impact at all, but it doesn't change anything.
The other common use case for the optical drive are DVDs and music CDs. Streaming content is making those less necessary.