Changes in grammar, punctuation, usage, and yes, even mechanics and layout are not trivial matters. They do affect how people read and perceive both the prose at hand the writer. However, precisely because they are not trivial, they cannot be handled as matters of right or wrong, correct or incorrect, rule bound and gagged. They need to be taught in the context of what the writer is writing and with patience. Sometimes the error is not so much about the error that appears, but is a symptom of something more important.
Some sentences fall apart because a writer jumps thoughts in mid-sentence.
Sometimes the error is not so much about the error that appears, but is a symptom of something more important.