It's not a problem, really - just that Nginx and Apache are very well suited for serving static content. Their mechanisms for caching, compressing, and request handling are very mature and efficient. The benefit is in reducing the number of requests to the rails app which is much less efficient in these matters. The effect is impressive in high traffic applications. For others, it may just be a matter of style or principle.

That said, someone running DCIY behind a reverse proxy would probably be aware of that switch. Maybe this level of detail is unnecessary in the README. In any case, I think we want to serve those assets from rails by default.