I heard today that there are many Japanese EFL teachers teaching at the university level who comment, upon encountering quite (or ludicrously) low level students' writing, that such writing "ruins" their (i.e. teachers') sense of English.
Shame on them!
Such a comment only proves that they do not at all understand what they are paid for. In this day and age, advanced students or highly motivated students do not really need a teacher because they can make use of a range of materials on the web, from news articles to grammer clinic to pronuciation training. The cohort who really needs a teacher is no one but those students who write at a hilariously low level, who have no clues for improving themselves without any external, or teacher, intervention.
Our primary job is to assist and lead weak students, not the cream of the society. That's what teachers are for.