I think that sentence diagramming has been unfairly criticized by
English teachers & researchers.  I reluctantly used it in a
course preparing students to teach English (Grammar for Teachers),
but I was pleasantly surprised at how well diagraming worked
as a visual representation of connections and purposes of the
indvidual parts of sentences.  I use Mark Lester's Grammar in
the Classroom, which is heavily based on conventional diagraming.
 
I would add, though, that the danger of focusing too much
on such a level of language is that students don't make connections
to meaning or to other grammatical issues.  At times, they
can diagram a sentence w/o understanding much about the whole
sentence structure or even the individual words.  In short,
diagraming is still just one tool with its own strengths &
shortcomings.  I'm sure you know that but I had to say it!
 
larry beason
Eastern Washington University