You sit there, marking up their essay with improper articles, run-on sentences, etc., and before you know it there's a legion of red ink-stains marching across the student's paper. Rather than a nesting ground for creativity, the piece has become a bloody battlefield for the instigation of proper grammar usage. There's no way the student will be able to remember everything you talked about, nor will you have time to discuss higher orders of concern!
So what do you do? Do you overlook the comma-splices, dangling participles, and other coherency issues in lieu of higher order concerns? The answer, unfortunately, is not as easy as yes or no. Sometimes those "grammar" issues do get in the way of coherence; sometimes those higher order of concerns do need to be addressed.
But to choose one over the other is folly. However, to teach grammar through pure memorization of "rules" is an exercise in futility. Language acquisition occurs when it is used for what it was designed for: communication. Those red ink stains are not helping with communication. They can help for some things, but to drown the student in red ink will not help them swim faster (I know, it's a mixed metaphor).
The concern with any paper should always be what's best for the student. If he or she appears to be following the assignment just fine but there are a couple problems with fuzzy grammatical constructions, then go ahead and discuss those. However, if their assignment is not being followed and you're getting caught up in the difference between "the" and "a," then something is very wrong.
Remember, "The task of the modern educator is not to cut down jungles, but to irrigate deserts." --C.S. Lewis