I think the point is that your product by definition breaks the browser by interrupting the users workflow. You product adds no benefit to the user of the website, and most people don't like it. If you made it opt in, those who enjoy what you're doing will be happy and you won't break the web for the rest of us.
We (CloudFlare) are in a pretty nice position, in that we can present a simple UI to our users (site owners) that allows them to fully configure the apps we serve without requiring them to touch any code. Giving site owners the option to present site visitors with a UI where they can opt-in sounds like a really novel and practical idea, and one I would personally like to experiment with implementing. Thanks for the suggestion!
