I'm more comfortable with "should" rather than "must" here.
My problem with "must" for the design patterns, is that the spec doesn't seem to be specific enough about the details of the design patterns to make them such a strong requirement.
Has this changed in the DITA 1.2 drafts?
And while "should" isn't a strong as "must" it is still pretty strong and much stronger than other options such as "may".
-Jeff