"I don't know why you teach diagramming. I've never used it in my adult life," so says about a gazillion parents who don't realize that not only have they used it, but they have used it every single day of their adult lives. Admittedly, some use it better than others, but they use it, nonetheless. Did you ever write a sentence that didn't make sense to you, and you went back and thought through what made it wrong? Yes? Congratulations, you used sentence analysis. That is, after all, diagramming at its core.
When I hear students say they will never use diagramming in the real world, I like to say, "I certainly hope not!" If you're still drawing out diagrams on paper in high school and college to determine if all the modifiers connect properly and you're writing what you actually mean, then I didn't do my job with you in middle school. Just like long division started out as a show-all-your-work proposition and later moved to mental math, diagramming begins on paper in order to understand the parts of a sentence and their relationship to one another, but it eventually becomes a mental process as well.
Have you ever wondered why teachers still cling to diagramming? Most instructors who still use it fall into one of two categories. I'm a weird one because I like both.
1. Diagramming strengthens writing skills.
These teachers see its usefulness in helping student unravel those unwieldy sentences that detract from meaning. When they see phrase after phrase connected in a string of modifiers, they can visibly see why their sentences have grown wordy. They can clean up the visual image in a diagram and at the same time, clear up the meaning in their sentences.
Eventually, this becomes a completely mental task, and students recognize wordy constructs more naturally. More often than not, they avoid writing them to begin with, eliminating the need to draw out the stick figures.
2. Diagramming strengthens analytical skills.
We start off our little ones with blocks and teach them to fit the right shaped block into the matching hole. Then they progress through Duplo and Legos to those fancy Bionicles. Each time, they think through the whole of a image, figure, or project but also have to break down the patterns to use individual pieces to build the larger structure without its falling apart.
Fast forward to high school, and all of a sudden we're asking students to do the very same thing with ideas, and we wonder why they can't do it. We've never provided them a transition from the concrete to the abstract, where they can see how ideas literally fit together in a concrete form. The sentence is a basic unit of meaning, and when we train students to analyze a single idea, we build a foundation for his future analysis of multi-faceted abstract concepts in his future academic ventures. In short, diagramming sentences is a transition between concrete and abstract analysis, and it trains the student to pull apart ideas, identify their parts, and reconstruct them more efficiently.
You may just want to give this old method a second chance.