Optical drives may eventually become less common. I admit mine rarely gets much use, but I still fancy throwing a DVD in there once and a while.
But that does present an interesting problem if you're, say, Microsoft. How do you sell Word to someone who doesn't have an optical drive? Would Microsoft or Adobe or anyone else who sells big-ticket shrink-wrapped software turn to the App Store to solve that problem? Not in a million years.
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.