I think anyone who uses English can profit from learning diagramming. Diagramming is logic applied to language. It REALLY REALLY helps when you start talking about the different types of clauses, and how to punctuate them, if you can visualize the pieces of the sentence and how they work together–that’s all diagramming is. A visual aid for sentences. It’s super easy to learn. For middle school, I really like Our Mother Tongue, which does have basic diagramming in it. We do much, much more in my home. Not only do we use it to help us with our English, but it is also really helpful when you get deeper into the grammar of a foreign language.
I have often talked about some of the things I have used diagramming to do. We DO use it to help our children learn important grammar topics, and it DOES help them. I also use it in untangling complicated seventeenth-century prose, and I have even used it to help me understand antecedents in a Biblical passage. There IS no easier way to teach the difference between direct objects and prepositional phrases, IMO. They just pop off the page when you diagram them! On top of all of this, it is just plain excellent for all visual learners, AND it’s really fun for the logically-inclined–like a puzzle. We sometimes have home “diagramming contests” where we take a passage of a song or poem or something and we all diagram it and see who gets the most right. (I think THAT would qualify us all as major grammar geeks.) In short, diagramming is a fun and incredibly useful tool for understanding our language, its rules and idiosyncracies, for improving writing, and for learning how the parts of sentences work together; it is also helpful when learning the grammar of other languages.