I'd always assumed that the product you'd build with one of these frameworks would live behind some sort of authentication such that SEO wouldn't really be an issue because the bots wouldn't see it. Your marketing site would be driven by a separate CMS.
I was working on a project that would have benefitted greatly from being single page, but the SEO requirement kept us away from it. (Going the phantom.js route seems wrong and icky, though Meteor is going that route so my tastes may be wrong.)
There are enough places where things can be handled with jQuery, but it's not nearly as elegant. The question wasn't if we were a single page application, but rather that we had enough in-page interaction that using jQuery was unwieldy. I have ideas that are coalescing on how to handle the in-between stage, but I'm not quite there yet.
What about situations where you need to be SEO friendly, but you have enough in-page interaction and reused components that mere jQuery starts to become unwieldy?
Nothing prevents you from using backbone in static pages. Also, Google's spiders can crawl javascript-heavy apps [1], but I'm not sure that's a future-proof approach. If your interactive content is that important for SEO, it should already be in the HTML, with the interaction layer on top.
