I like the apologetics for node.js in the first section. a similar argument for why you are using jruby instead of node.js for the "backend whatchamajigs" would be nice.
Workers can be written in any language, which was another major design consideration. We have about a dozen types of worker right now; when we want to evaluate a new language, we can port just one. Alternatively, we can write any new worker types in an arbitrary language.
Thus we're not married to JRuby.