Writing DTDs is not an easy task, and enforcing strict guidelines for smaller documents like letters, memos, faxes etc. is not essential. (An exception would be a legal obligation to record correspondence in letters). Well formed and valid XML documents (see Well formed and valid documents) exist specifically for those documents where developing a DTD would be overkill, or for Internet delivery of documents.

Validating documents is an editorial activity that makes sense at the author or publisher site, but not at the user or reader's site. Therefore validating documents after they have been delivered is ineffectual. Once documents have been accepted for publication, they should be correct. If they are not, it is not the user responsibility or fault. This is especially relevant on the Internet where download times are an issue.

The choice is not all or nothing. It is possible to switch between valid and well-formed for a single document. It would make perfect sense to create documents, validate them against a DTD and publish them as well-formed without the DTD.