This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
Well, basically the purpose of validation is to determine whether the document is valid. That's all. It isn't like a compiler which tries to identify all errors, it just says "yes" or "no" to the question of whether the document is valid.
As I said, if your users are having problems creating valid documents, it would be better to provide them with tools that make it easier for them.