Bryan Caplan has a very good post on the human capital and signalling models of education. The key point is this, under the human capital model someone who forgets knowledge is no better than someone who failed to learn the same knowledge. Under the signaling model, however, failing and forgetting are very different. Bryan illustrates:
If I’d failed Spanish, I couldn’t have gone to a good college, wouldn’t have gotten into Princeton’s Ph.D. program, and probably wouldn’t be a professor. But since I’ve merely forgotten my Spanish, I’m sitting in my professorial office, loving life.