Some aspects of English, such as grammar, spelling, and vocabulary, have right and wrong answers. Grading is a good way to determine how well a student understands these rules of the English language.
For some, English classes teach a new language (in countries where English isn't the primary language). Hence, it should be graded, at least in these countries, to determine how well a student has learned the specific language.
grading is sadly one of the only ways to make kids pay attention in classes. Other ways include making the classes more entertaining and interactive, which some teachers have accomplished, but is very difficult.
it is far easier and more effective to grade people on the 9 pillars of intelligence; then offering advice on what professions somebody might do best with their scores.
Since majority of the time all the answers are supposed to be "your intrepretations" a teacher can't be responablly asked to judge your opinion and give a grade on it