Why you can afford to forget useless information after an exam:
According to the human capital model, failing (i.e., never knowing) course material should have exactly the same career consequences as forgetting (i.e., no longer knowing) course material. Either way, you lack the skills – and the labor market should treat you accordingly. According to the signaling model, in contrast, the consequences of failing and forgetting can totally diverge. When you fail to learn useless material, you send a bad signal. When you demonstrate mastery of useless material, you send a good signal – whether or not retain what you learned. Employers naturally snub people who fail, yet smile upon those who merely forget.
The Dish 