I think the argument React.js and other component-based frameworks like Polymer and Brick are pushing is that you should always be building your application out of self-contained components that have a defined a defined set of inputs. They would argue (and I would agree with them) that "building something that's not" is a code smell.

This (in theory) helps code maintainability. The challenge then becomes designing components in such a way that code paths do not need to cross lines between these things.

I'd agree, except that I consider the level of granularity to be too fine the way React does it, resulting in too much boilerplate to do simple things.