It can be a good thing the way HTML-CSS-Javascript separation can be a good thing. In the case of Clojure, it actually is possible to write the layout code in a declarative style and keep the handlers completely separate, but for most of the popular languages, it's easier to just use Qt Designer, Glade or similar to generate an XML layout and write the handlers in your language of choice. Compared to the old approach of GUI designers generating a pile of C/C++ code, which you then modify, the XML blob is actually much nicer.

Also, most of the XML-presentation libraries offer multiple language bindings, so there's less of a requirement for your language to provide its own GUI toolkit, and it's easier to convert your app from one language to another (e.g. injecting Python into a legacy C app) without losing any of the details from the original interface.