There is value in the ability to produce correct English 'off the cuff'. You could argue essays are the best way to get students to produce off the cuff written text. Hence, it makes some sense to ask students for essays, and then judge those essays only for form.
However, it is rather important that students know their essays are not judged as essays, but only judged on the content. Otherwise you teach students that form trumps content in essays.
When judging an essay as an essay correct English barely matters. What matters is how convincing you are, and how interesting of a read the essay is. This is a great skill to have, and testing it also makes sense. Really though, we should separate these two forms of testing.
However, it is rather important that students know their essays are not judged as essays, but only judged on the content. Otherwise you teach students that form trumps content in essays.
When judging an essay as an essay correct English barely matters. What matters is how convincing you are, and how interesting of a read the essay is. This is a great skill to have, and testing it also makes sense. Really though, we should separate these two forms of testing.