If the purpose of a grade is to communicate specific information about what the student has learned, then the letter-grade system is far too blunt and subjective to offer any useful information about what they've achieved and where they need to work harder. In an Atlantic article, Jessica Lahey argues that points-based grading is a subjective measurement masquerading as objectivity. She writes:
"I realized that the current system of points-based grading is highly subjective. As Alfie Kohn has written, 'what grades offer is spurious precision—a subjective rating masquerading as an objective evaluation.' A few years ago, I told my students about a study I’d read that showed judges rule more favorably after breaks, so from then on, students left snacks in my office and reminded me to take breaks when they knew I would be grading their work. If the purpose of grading is to objectively evaluate student learning and achievement, surely my work breaks and snacking habits should prove irrelevant in their calculation."
http://www.theatlantic.com/education/archive/2014/03/letter-grades-deserve-an-f/284372/