reno,
I did not intend to say that preparation for the in-service would be without merit, for it will definitely help with the repetitive learning requirement; my point is that the mock boards do not correlate perfectly with the real thing for the very reason that you point out (someone can be at the 50% on the mock boards and not perform nearly as well on the real thing; likewise, someone can walk in to each of the mock board exams without special prep and perform significantly better on the real thing with adequate prep). Also, there is a difference in the overall "quality" of the questions between the in-service and the ABD exam.
My chair was on the ABD for years and involved with the writing, screening, and selection of board questions for many years; my thoughts reflect his input over the years. The questions used on "mock boards" were often either first-time-through questions or those rejected from the real thing; while the general subject matter covered is similar (folks tend to write questions based upon their respective area of expertise, therefore any questions they submit tend to be similar... and there are relatively few people writing the questions), the quality, clarity, and relative importance of the question asked will differ, on the whole, from their "real deal" counterparts.