I'm sure they've discussed this at length: what is the incentive for a user to annotate case law?

Law happens in such slow motion and not exclusively at the computer that you don't really get the feedback as you would in programming (fixing a bug versus arguing a motion) - which is one of many reasons why Stackoverflow is successful (not that this is really a fair comparison anyway - more of an example). If my immediate success on compiling is dependent on trying out one of these suggested solutions, I'll certainly have an incentive to comment, upvote, and potentially contribute later on knowing how much it helped me.