It's okay if you disagree, but experiment with it yourself first. Different dogs tend to depend on different cues - which is why one training "system" doesn't fit all dogs. With the main group of competitive dogs I train with, 3 rely on shoulders, one relies on hands, and one relies on hand movement. None rely on movement from the waist down, and none rely exclusively on voice - voice is a secondary cue and a "back up system". This is a great advantage for distance work, because it doesn't matter where my feet are headed, as long as my shoulders are pointed towards the obstacle I want them to take, they're fine. This is why some of the best NADAC handlers can achieve such spectacular distance, with minimal vocal signals and minimal movement - if motion was the king, they wouldn't be able to do this.
Also consider how much dogs rely on body language to provide information about their handlers and other dogs, vs. how much they rely on vocal information. Obviously they rely far much on body language, so wouldn't it make sense that this extends into agility? As humans, we tend to rely on verbal information, and try to extend that to our dogs instead of accepting that they rely far more on body language, and using that to our advantage.
At least once per training session, we run courses with absolutely no verbal signals allowed - try it yourself - it's AMAZING how much better our dogs run without the verbal cues. Try it yourself a few times - it's lots of fun, and will really force you to reconsider how you handle your dog. I sure learned a lot from it
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