I was just speaking w/ my grad students on the role of metaphor
in language & rhetoric, and it occured to us that sentence
diagramming may simply be a metaphor for describing a complex
concept (the hierarchy & interrelations of words in sentence)
in terms of a simpler concept (visual representation of
lines, diagrams, etc. found in sentence diagramming).
 
While I stand by the assertion that diagramming can be useful,
perhaps the problem w/ diagramming is that the metaphor doesn't
work for many students.  Metaphors only succeed w/ a group of
people who can understand one concept when it is expressed
in terms of a more familiar, less complex concept.  One problem
w/ diagramming is that it is not very familiar, and often
students forget that it is nothing more than a metaphor.  Hence
the lack of sentence diagramming in helping students (SOME
students) understand language.
 
Thought I'd share this view while it's still fresh (but perhaps half-baked
to continue that metaphor).
 
larry beason