decent_exposure helps you program to an interface, rather than an implementation in your Rails controllers.
Sharing state via instance variables in controllers promotes close coupling with views. decent_exposure gives you a declarative manner of exposing an interface to the state that controllers contain, thereby decreasing coupling and improving your testability and overall design.
It’s almost as if the program were composed out of some kind of discrete “objects”, which only communicated with each other via well-defined “interfaces”, and “encapsulated” their implementation details. It’s like a whole new paradigm of programming!
(Just in case anyone misses the joke, this is not an indictment of decent_exposure. It’s a commentary on just how broken the Rails Controller/View interface is and has always been. I find it ironic that one of the most OO languages in modern use, the notion of a public method interface has to be hacked into Rails controllers with a special syntax.)