Replying to a Twitter conversation with @johnjoseph_code:
Our number one concern is helping our developers get better performance out of their app. Period.
We are recommending Unicorn so that developers will use more RAM. Most Rails apps that I examined were using thin - because we had recommended it in the past - and were not using anywhere near the full capacity of their dynos in terms of memory or CPU. Because of Unicorn's forking process model, it is often a trivial switch for an application to make that provides a near instant boost in performance for them by actually using all the RAM they have requested. With 2X dynos, developers can verically scale pretty well, generally fitting 4-8 unicorn workers on a dyno (or more if you set extremely tight conditions with https://github.com/kzk/unicorn-worker-killer).