I’m one of those lucky students who never had to diagram sentences — most of the English instruction I had (in both K-12 and journalism school) focused on specific rules and guidelines that people tend to have problems with (its/it’s, who/whom, passive voice, etc.).
Regarding K-12, I’ll go ahead and offend every single PBC reader — and defend this. People possess a language instinct and, at least in their native tongues, do not need to be taught how each individual part of speech functions (though it does help to learn the parts’ names, of course). Time spent on sentence diagramming, which I suspect just frustrates those with low language ability, can be better spent identifying common problems and teaching students how to fix them. (I’m not saying that schools are doing a good job of this, just that it’s a reasonable way of approaching grammar.)
One of the few complaints I have about my journalism-school experience, however, is that while it added to the list of rules I follow, it didn’t really drill into my head the knowledge one needs to diagram sentences. This knowledge is useful for professional editors (and to a lesser extent, writers), so recently I’ve been making up for lost time.
But I still don’t find diagramming useful. Whenever I try, I spend far more time learning the rules of how to draw the ridiculous picture than I do parsing the sentence. I find it much more helpful to go through a sentence word by word and clause by clause, naming the function each serves.