Not to defend this practice at all (I'm horrified). But there was an interesting study some months back where they compared (1) a group of students who were told to favor quantity over quality, churn out tons of writing, and then select their best for grading; (2) a group of students who were given feedback to hone a smaller amount of writing. At the end, (1) was determined to produce greater gains in writing than (2).

Again, not defending this-- I am eternally grateful to those teachers who helped me learn to write-- but just noting that teaching writing is a tricky business.