Hark back to the early home computing days when the apps that came with a computer were built/owned by the operating system vendor. Now think about the product landscape and OS advancements that have taken place over the past 20 years. Those advancements and innovations were purely a function of the underlying operating system opening up it's APIs, and supporting the needs of application developers. The result has been better products for the end user, and billion dollar software industries.
Now consider web browsers the new, modern, operating system. They only way to see the same kind of innovative explosion in the browser space is through extensibility. Netscape/Mozilla have supported cross-platform extensions since the beginning. Microsoft has wedged it's proprietary add-on models (leveraging highly proprietary technologies from COM/DCOM, to ActiveX) into IE. Apple, in a series of miscues with Safari, doesn't have an extensible model for their browser. It was a joy to see Mozilla finally take add-ons seriously once they sput out of AOL. While Netscape had a framework, they didn't provide a clearinghouse, so add-ons (plugins as they were called) had limited success as an industry early on. Mozilla finally seems to be getting it right with http://addons.mozilla.org; though there's room for improvement.
I'm really excited to see some friends pulling together something we'd talked about years ago; addoncon (http://addoncon.com/). The fact that industry hasn't been pulling together on the extensibility front to date, is beyond me. I'm planning on attending, and if you care about how the network evolves, I hope you do too.