I'm not disputing that bulding it into the OS is a big deal, but pointing out that in its 2 years of public release on the app store, Siri didn't see the organic growth of apps like Instagram or Hipstamatic.
Apple has added new features to integrate it more deeply with things like the Calendar and Reminders apps, but at the same time, half of Scott Forstall's demo commands at the 4S keynote were exactly the same as the Siri demo at 2009's All Things D conference.
A big part of building it into the OS is that it only takes a single action to activate, instead of the half dozen or so plus a considerable amount of waiting that it takes to activate the standalone app. I believe this will result in a qualitative difference. The app was mostly a novelty that was usually slower and less convenient than doing things directly. The OS service is likely to be considerably faster and more convenient than the other paths that exist for achieving what it can do.
Apple has added new features to integrate it more deeply with things like the Calendar and Reminders apps, but at the same time, half of Scott Forstall's demo commands at the 4S keynote were exactly the same as the Siri demo at 2009's All Things D conference.