You're in law school, right? Most of our professors here are humble enough to tell us that law school exams test a certain narrow range of skills, not ability to understand the law or function as a lawyer. Indeed, they constantly say stupid things like "Grades are not very important" (Yalies!). I'm sure this is, in part, a self-interested and cynical statement intended to shut down requests for grade changes and such, but I think there's plenty of truth to it.
On the other hand, sure, I can imagine that issue-spotting is a key skill for attorneys (though not for associates in their first several years of practice at biglaw firms, alas, since document review and such really don't require this kind of insight). But unless you have one professor for more than one semester, in a typical exam-based class, you are not able to adjust to learn how to "jump through hoops and judge what an [s/he] wants" on the exam. In the firm, one has much more time to learn, through both trial and error and (more important) a network of knowledgeable peers and superiors, how to please those senior associates and partners. Being able to listen to social cues, accept criticism, and adjust one's behavior accordingly are all very different qualities than those that lead to superior performance on an issue-spotter. And if you've ever read model student answers to law school exams, you know that the writing quality falls far short of that one might seek in a legal memo.
I'm not saying grades don't tell you something. I'm saying that they may not tell you all of the things you need to know about someone before deciding to ask her to join your firm. Again, I haven't seen any good research on this, and I'm always open to evidence that I am wrong. I think looking at the data for other associates with poor grades and for black associates with good grades might help us sort this out.
Also, I'd be interested in your thoughts on whether top students at lower-ranked law schools might be better recruiting candidates for firms, since you seem to believe that the emphasis on GPA/class rank makes sense.