It sounds like the real benefit is not performance but the simplicity of having the slow-client spoonfeeding built in rather than requiring an external Web server.
a.) they could achieve that a lot simpler by bundling nginx, Unicorn, Rails, and a pre-vetted set of config files and shell scripts to bring the whole thing together and
b.) that's the value proposition of PaaS offerings like Heroku. Heroku is pretty damn simple already - just git push your code - and you'd outgrow it around the same time as you'd outgrow the bundled slow-client spoonfeeding, so what's the value proposition of this?
