I think restricting access in this way will slow the growth of browser extensions/addons as platforms for launching full-fledged web services and apps.

I see more and more web apps relying on extensions for a core part of their functionality, and this is only going to hurt them.

For installing non Web Store extensions, I would rather see an extra warning message or notification, rather than a protracted process that many users wont be able to figure out. Extensions are good, but not if they're hard to install.