The thing is, mixins are a pattern, and patterns are interchangeable. So you can use composition, single inheritance, multiple inheritance, and mixins interchan

The thing is, mixins are a pattern, and patterns are interchangeable. So you can use composition, single inheritance, multiple inheritance, and mixins interchangeably in most cases. It's not that you need a particular pattern, it just might be easier to express a certain idea with a given pattern. I agree with you that composition is easier to understand, since component APIs are well defined. For large teams like those at FB, this simplicity can be a big win.