That Arban exercise is part of my daily routine, it is very beneficial. My teacher advocates plenty of jaw motion for a relaxed modulation of shape. Greg Spence argues for the concept of shape (see the sticky Trumpet traps, frustration etc) but I don't recall him advising to consciously minimize jaw motion. So far I am very impressed with Greg's stuff, even only what he puts out for free, which is already a good bunch of instruction and contains the important stuff. It has been more helpful to me than anything I have ever seen in writing or over the intertubes. In fact it's the only info of that kind that I have ever found helpful.