Because we used it for our funding announcement at the end of last month -- a full press cycle, where the Andreessen Horowitz and Matrix Partners press machine pushed our story out to all of the tech blogs, with all of the traffic that that implies -- and not only did it work fine, it got us to #1 on Google for "meteor." Above, you know, actual meteors :)
It works fine for us as a stopgap measure and we wanted to share it with others. We put it in a optional smart package that isn't included in new projects by default.
We're near the end of a major rewrite of Meteor's page update engine. You can see the latest progress on the 'spark' branch. One thing that happened during this rewrite was the conversion of Meteor's templating to be 100% string-based. Check it out: go to meteor.com/faq, open your browser console, and evaluate "Template.faq()".
This means that the server can render the templates for your app without having a DOM implementation of any kind (much less a headless client.) 'spiderable' doesn't do this yet, but it will by Meteor 1.0 (if 1.0 even has a separate package.)
For the record, the ratio of our investment in auth and accounts, to our investment in Spark, is about 2:1.