Consider the following scenario: YouTube releases a suite of new features, but they require some new functionality from HTML5. This is fine for Firefox, Safari, Opera, etc., but if you're using IE it'll prompt you to install Chrome Frame, just like it prompts you to install Flash. I could see it getting to the point where people instinctively do this just as they do with Flash.
Hell, maybe it could even be used to subvert Flash itself. Suddenly IE users everywhere are viewing <video>-based YouTube videos.