I met with Nathaniel at this year's OOPSLA conference. He's very enthusiastic about its benefits. He's been working to integrate support for this organization of the libraries into Squeak Smalltalk as a demonstration.
Essentially, the main benefit is that it is effectively optional for the programmer to be aware that the Traits are used to organize the code instead of just classes. Also, dependency information is tracked. There'll be a Squeak image released in the next month that shows this and all the core libraries re-organized on this principle.
I'm also interested in acheiving the same kind of behavior for the Traits of my Smalltalk derivative, Slate.