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Mark, for our WordPress client sites and our own sites, we're implementing AMP on all pages on the site(s). What we know is that Google's implementation (adding the pages to the results) is changing, and we don't know what the rollout plan is in search. We started only seeing it on news sites, but it's been expanded. I do recommend making the pages available on sites if it doesn't take away from the content on the pages. So, if the content of a page relies heavily on images, for example, it might not be appropriate to use AMP or have an AMP version because it really isn't going to be a mobile friendly page. But, if it's primarily content-based or text-based, then you would want AMP to be available on those pages.