Not an "app store" in the sense of the iPhone app store, but an advanced software packaging tool (e.g., Debian apt-get). Think of Software Update on steroids: can be used for all software on your machine + an interface to add third party apps from a repository.
It makes for a superior user experience: less searching the net. There would be less page views to find software, so perhaps a reason to drag feet on such a feature would be keep software search page views high (see what good that did for MySpace).
Right now we have: Software Update + searching the web for software + useful widgets like "Application Update" to keep 3rd party software up-to-date + port command for MacPorts + update feature inside apps + ...you get the picture.
With Ubuntu, for example, all you really need is one place for everything. It checks for dependencies so orphaned packages are removed. It seems to allow for better common package sharing.