"[S]imply telling students that their answer on a test is wrong or right has a negative effect on achievement... The best feedback appears to involve an explanation as to what is accurate and what is inaccurate in terms of student responses. In addition, asking students to keep working on a task until they succeed appears to enhance achievement" (Classroom Instruction That Works, p.96).
I know the cons. The kids don't read the comments, anyway. Or, with multiple subjects and multiple classes and multiple assignments, this is almost impossible.
First, in most cases, we've conditioned the students to ignore their graded assignments. "Here, put it in your binder. In the appropriate pocket!" We simply need to take time to review every assignment. Are we giving assignments to teach or to collect grades? Next, with all of those assignments for all of those students, comments would take a ridiculous amount of time. So, we can do two things: cut down on the number of assignments that we give and/or create rubrics for every assignment.
One last thing... If we're giving assignments that are just so easy that comments aren't really necessary, we should rethink the quality of assignments that we're giving to our students.