Context beats consistency. Reuse only works well when the particular instances are so similar that you're willing to trade the small differences for the increased productivity. That's often the case for infrastructure, such as Rails, but rarely the case for business logic, such as authentication and modules and components in general.

- David at Loud Thinking.

A framework is a framework, no more no less. Its downfall is the inclusion and creep of business logic - no matter how implicit (e.g. authentication) - into its domain. On the broader question - is re-use overrated? - i am always wary of speculative generality and counter-productive reuse in code. As one commenter to David's article points out,

Programmers have an irrational fear against throwing code away.