I think, there are various people using XML who like having XML data without any validation. I'm a strong proponent of having validation nearly always when using XML. Comparing the situation with RDBMS data, would make this clear I think (I don't mind proving things about a technology, taking cues from another technology which is hugely popular). Do we ever use data in RDBMS tables, without the schema (we don't)? The same should apply to XML, since validation is very closely defined alongside XML (DTD at least, and then XSD). If DTD or XSD is provided along with XML parsing, by the XML toolkit of choice, then why shouldn't we use validation whenever we're using XML -- as a consequence, we're working with a better design?
Interestingly, validation doesn't always happen when using XML, because it hasn't been made mandatory in the XML language (like schemas with RDBMS). People using XML, sometimes like having XML data quickly transported between components or stored locally -- and they don't use validation in the process; which is fine since it meets the needs of an application.
Sometimes, people using XML are influenced by how JSON is used. Presently, JSON doesn't has a schema language (but I came to know, that this may change in the future), and JSON is very popular & useful for certain use cases. Therefore, people try to use XML the same way -- i.e without validation.
