See the CCH recruitment office. DTDs should be as tight as possible. But not all documents fit in these tight DTDs, so we need a lot of DTDs. We need a system to keep them related. We want content experts, not SGML experts to adapt a DTD to a specific need.
Switchboards make that possible.
The only way a user can change his DTD is by modifying the switch file. The advantages Only foreseen and approved content models can be created. But it is not to the regulator. All mistakes, misspellings, misnumberings, misstructures must be preserved. Laws have long life cycles.