Google seems now to be suggesting that is exactly what they want, for webmasters to tell them what pages are the most important, and not just through links. If you check the sitemaps documentation on the webmaster site, there is a blurb about using the priority tag to indicate the relative importance of a page - although Google does stress this won't affect your position in the SERPs, it might have an effect on crawl rate in much the same way IBLs do, and could have other effects such as determining keyword relevance (page one is optimized for foo, page two is optimized for widget, but you tell Google page two is relatively more important than page one, so Google thinks you care more about widgets than foos).

Just another internal factor to add to the balancing act and list of fun things to speculate about?

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The best way to learn anything, is to question everything.