You can't program the client in Java.
That somebody can do both things should, in my opinion not mean that he has do both things.
And that is how it's relevant. Any developer who can't grasp this concept has no place working for a company producing web-based software.
How many Java backend engineers know how to unit test javascript, how to make modular templates (e.g. writing their own tags, or nesting/tiling their JSP's), how to systematically minify their code in production and use CDN's ... or even how to use any design patterns in Javascript?
(But who wants to is a separate question.)