> Most of them are never used simply because their use cases aren't common
> in regular writing.  OTOH, structures that ARE commonly used have been

That's not strictly true of CITE or DFN in my view.

A more accurate statement would be that they are not commonly used in 
advertising copy and authors see inadequate benefit in using them 
over the presentational markup which produces the same effect in English
language visual or hardcopy output.  As I think we want to discourage
presentational markup, this latter reason ought to be considered an
invalid counter argument.

The DFN concept is actually heavily used in Wikipedia, but Wikipedia
has a somewhat confused use of HTML in that it is largely used as a
presentational intermediate format for rendering, even though you can
pass through structural HTML as well.  Wikipedia's standard rendering of
the DFN semantic concept is bold (rather than the traditional italics)
and it is achieved by either recognizing a link to the current page,
or by explicit use of the Wikipedia language's equivalent of STRONG.