Mixing vocabularies. My young friend did not realize that you lose a lot of the value of using a public vocabulary when you pick and choose snippets from one vocabulary and another and then mix them together. You can make a real mess by copying and pasting declarations from one schema into another totally disregarding those pesky namespaces.

The reasons that people got together to create those public vocabularies is so that they can easily and conveniently interchange documents, so they can share in the costs of customizing tools and other document-related infrastructure, and so that they can move from collection to collection and organization to organization without having to start all over learning an unfamiliar application. Also, because habits take a while to develop and once you have them it is easier to work in an environment in which those habits work than to break old habits and form new ones.

By mixing and matching from among several public vocabularies you are walking away from all of the traditional advantages of using them. You will create documents that no one else will be prepared to receive or process, so you have no interchange. You will not be able to use tool customizations or stylesheets that have been developed by any of the use communities. You will not be able to take advantage of the economies provided by using vendors that already know your requirements, nor will you have the built in quality checks that come of vendors knowing your vocabulary.