I think browser plugins are a dying breed. Not so much because of what you can do with HTML5, but also because we are so strongly moving towards an app based future. You can do a lot more with a native app consuming, processing, and displaying the HTML and XML from the server than you can with the browser.
People like apps. Developers have proven that its not completely painful to support a number of platforms. Most importantly, many sites and content producers can monetize the app in a way they've never been able to via a straight web browser experience. 0.99 -> 1.99 an app isn't much, until you're selling hundreds of thousands of copies or more, and suddenly its quite a lucrative approach to delivering your content.
Ask Rovio if they made the right choice by writing their flagship app native or using a plugin like Silverlight or Flash and targetting the browser.
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-Scott
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