Finally, weighing the time it takes to develop this framework vs. the probably much smaller time one takes to do so while in law school, as I think JamesDean pointed out, is another possible reason why 0L prep is not worth the hassle. So I'd be curious to hear your take on this.
The way I see it, a way to do well on a law school exam is to have a very lucid understanding of the law within the scope that your professor presents the subject matter and know how to apply the law to the facts, being able to recognize and fully exploit issues that arise.
0L substantive prep could give you a bit of a heads-up on forming this lucid understanding. I don't think that much inefficiency results because you're less equipped to comprehend the material: if you start from the beginning of the supplement and sequentially proceed, you shouldn't get lost. Your Contracts professor will not imbue you with the power to understand the concept of mutual assent, for instance, as if before the concept was mystically inaccessible. The inefficiency will result from covering areas beyond those your class will cover- you have no way of knowing what your professor will omit from his or her class.
And this inefficiency in addition to the possibility of forgetting the stuff before class begins makes it reasonable to conclude that it is not worth the hassle- the returns are definitely not as great as general prep, such as GTM and LEEWS.
But it's also reasonable to conclude that substantive prep is worth the time, if you're so inclined. Just reading through a supplement from start to finish, without outlining or taking notes, can give you a good idea of the topic. Then when you are reading your casebook and attending lectures you will have a bit of a grasp on the essentials so you can focus on your professor's perspective on the law and the aspects that he or she chooses to emphasize.
Here are a couple threads which recommend supplementsviewtopic.php?f=3&t=201636&p=6270766&hilit=Supplements#p6270766viewtopic.php?f=3&t=194417