@scheme on <keywords> allows for reference to an external or internally defined keyword scheme used for the various <term> elements inside <keywords>. I'm interested in attaching keywords to a TEI document that are not from a controlled vocabulary, such as author-defined keywords. In such a case, I would be inclined to do:
without using @scheme. Lou agreed with this approach on TEI-L:
In fact, P4 did not require @scheme, so we would be going back to the glory days.