As a developer I'm in favor of this. Just take a look at the how popular the question of "How do I enable Reader mode" is on SO[1], and how complex and mysterious the actual algorithm appears to be[2], and it's evident how authors and implementors alike could benefit from a dedicated element.
Although, there's the question of how similar in definition it'd be to <article>. Is it merely a more specific version of "a self-contained composition [...] that is, in principle, independently distributable"? Or is it a more specific <div>; a semantic "wrapper" element? If it's the former, this could just as well be an empty attribute, as <article main>. Not too different from ARIA, which maybe makes it a little redundant, but it's less to type in CSS (article[main] vs article[role="main"]), and also achieves landmark parity without breaking "legacy" HTML parsers, frameworks, etc. which only expect <article>. If it's the latter, it probably makes more sense for it to be an element, where it wouldn't say whether the content was self-contained or not; just that its contents are considered the primary focus of the page, except anything that would otherwise be excluded in the document outline. [1]: http://stackoverflow.com/questions/2997918/how-to-enable-ios-5-safari-reader-on-my-website [2]: http://mathiasbynens.be/notes/safari-reader