In chapter 3 of Cognitive Surplus, Shirky discusses the work of Edward Deci, which showed that people paid to solve a puzzle spend less time working on it than volunteers performing the same task. Although Shirky isn't discussing this in a education context, the findings can easily be applied to assessment and feedback.
Extrinsic motivations are those in which the reward for doing something is external to the activity, not the activity itself... extrinsic motivations can actually decrease intrinsic ones.Shirky does go on to discuss the role of positive feedback as an intrinsic motivator, whereas the "payment" of "marks" works in a negative context.
Translation: Let's stop over-assessing students and making them dependent on external motivation. Love over gold.