It would be trivial to create a q.series or q.waterfall abstraction. The real benefits of promises are that we get back `return`, `throw`, `try` and `catch`, not that it makes our code 'look' more sequential, which is totally possible using callbacks and async.js.

Waterfall is an easier top-level API that hides a clean low-level API. When it's all you need and it works well then you won't want to switch. When it breaks or you want it to behave better then a nice low-level API is beneficial.