This is related to this quote by Michael Kay on the XML-DEV mailing list, on the subject of validation.

There is also scope for reasonableness checks to catch data input errors. But they belong as close to the user interface level as possible, not at the information management level.

Which is fine as far as it goes but the use of the term ‘user interface’ is misleading, I think, to most people (myself included) this implies ‘end user’ but this is not always the case. If, for example, you are writing a service (web- or otherwise) for external, or even only internal, use, then the ‘user interface’ is the service interface that you expose and it’s perfectly reasonable (in fact, I’d argue that it’s pretty much essential) to validate every message that your service receives.