Reducing elements and attributes makes authoring easier because you eliminate the noise you may get from a default DITA deployment. Don’t use <screen>? You can eliminate it. Fewer elements also means fewer styles to maintain.
Best of all, constraints get built into a plugin, which means you can have any number of constraint sets active in your toolkit. Want a set for experienced DITA authors and another for SMEs? You can do that. In fact, you could reduce DITA to just paragraphs and lists, making it simple for non-DITA users to begin creating DITA content.
I’ve posted a constraint plugin to sourceforge to provide an example or a starting place for others. The constraints are based on the checks run in our QA plugin. Installing the constraint plugin is straightforward and I’ve included a few sample topics (with appropriate doctype declarations); setting up Oxygen is the easiest way to test them out. You can configure XMetaL to use constraints too, but it isn’t nearly as easy.