This week's book giveaway is in the OO, Patterns, UML and Refactoring forum. We're giving away four copies of Refactoring for Software Design Smells: Managing Technical Debt and have Girish Suryanarayana, Ganesh Samarthyam & Tushar Sharma 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.