SEO was our first concern, but then we realized performance was the big win. Serving the user a full page of HTML rather than waiting for JS to download before rendering HTML makes a big difference, especially on high-latency or low-bandwidth connections (mobile).

EDIT: We decided against using something like PhantomJS or node-chimera because why deal with a DOM if you don't have to? DOM is complicated and slow, compared to string-based templating. Plus, it's another moving part, another process to keep up and running. The Meteor guys aren't too excited about the PhantomJS approach either, and IIRC they'd like to be able to serve HTML without firing up a DOM.