The less options, the less choices and the less time spent in choosing or making wrong decisions. Conditional style sheets are easier when there are fewer options. If you have one DTD for all documents, all other documents loose some control. If you have lots of specific DTDs, it is hard to make global changes, and ensure compatibility between the DTDs and Docs. A DTD is defined by the set of documents it allows.
More complex changes require rewriting parameter entities that define content models.
Then the central switchboard sets all other switches to the appropriate value. Then parameter entities with all possible attribute values are declared. Then external modules with all element and attribute declarations are called. Then all references to globally ignored elements are removed and the resulting content models are normalized. If necessary, we add a switch.