Robert, thanks for expanding the dialogue by disagreeing on diagramming sentences (you are far from alone). The important point, though, is that diagramming is just a metaphor (or perhaps I should say synecdoche, now that those old English classes are on our minds) for understanding the parts of speech. A freshman composition instructor told me recently that it’s hard to explain to students today what’s wrong with their writing because they don’t know the components of a sentence or their relationships.
How can you explain a sentence fragment to a student who doesn’t know that a sentence needs a subject and a verb? How can you correct a dangling participle if a participle is no different from any other word? How can you distinguish between the past and past perfect tenses if the concept of verb tenses is unknown?
When I attended junior high (no middle school back then), the fundamentals of grammar epitomized by diagramming were already fading. Fortunately, French and Latin helped clarify how words change in different contexts (conjugation and declension). But I don’t think students study foreign languages much, either, any more.