Yes, marks can measure merit, because they measure how well a person did on that particular form of measurement. The person who gets the best score on a test is not necessarily the person who knows the most, but it is the person who demonstrated that they know the most by scoring the best on the test. This is not perfect, but it gives a rough idea of merit.
Marks for children and devices that measure success will help in many ways. Most teachers in the United States will tell you that technology in the classroom, though difficult during learning stages, helps out in the long run. In this sense, marks, as long as they don't over extend the opinion of the teacher, are good.