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.
