The problem, as you have pointed out, with case studies is that people don't know how to use them correctly, not that they don't have value. If marketers understand how to scale and how to pull out the strategic structure of why the case worked, they can move the pieces across to their own strategic plan, if the pieces fit. In my experience, the problem with our profession is that not enough people understand or use strategy to begin with so they can't replicate the pieces in their own campaigns correctly. We are desperate to use the copy & paste to achieve similar results because we don't know any better.
And just a thought about your comments on reach (retweets and fan numbers). There are places where reach is an important goal, but it can't be the end-all, be-all. If you're shooting for top-of-mind, as Esurance was, increasing fan numbers is a first step. But if you quit there, you fail. Marketing is a process, not a one-time act. Reliance on case studies alone will not produce success. But as an idea generator, I think they are valuable. The key is in learning how to replicate correctly, or whether to replicate at all.