I do totally disagree. What if your economy of motion is different from mine? Just think: your arms, your hands, your fingers are differents from mine. It just happens that people are different. One is fat, one is slim the other is tall, another is short. There isn't a definitive method for everyone. This is what teachers try to teach you. And they are all wrong and they all play like monkeys.
I think this is where this diagram says a lot nicely:
We may all be different sizes and shapes but for the most part we're in proportion and our tendons and general physics for movement are the same, I'll be basing all things here on the presumption that none of us have odd deformities or mutations.
As Mystique mentioned with the left hand thumb being behind the neck as opposed to over: your stretch will be improved no matter what shape your hands are, even if someone with larger hands will have a naturally larger stretch in both scenarios. Some people believe that if you have large hands your thumb will have to be over the neck cause you've got a bigger thumb, however, proportionately you'll have longer fingers so, though your hand will be further below the neck of the guitar, your fingers will still have the reach required.
As well as the reach factor, the thumb being over the neck can cause finger independence issues. These can be overcome with practice but it's far easier to move the thumb behind the neck.
Another argument against left hand thumb position being over the neck is that it causes your fingers to scrunch together more (for similar reasons to those that cause your restricted reach) which leads to increased tensions in your hand.
With the thumb behind the neck, the way you fret notes and the positioning of the fingers remains the same no matter what string you're on. The angled nature of the thumb over style means that your fingers will naturally reposition as you move onto different strings. This is detrimental to accuracy and causes unnecessary tensions in the hand (which in part contribute to the finger independence issues), and obviously makes guitar playing harder due to having to fight against the tensions.
Finally in terms of the thumb: if you have it over the neck your wrist is less likely to be in a straight position. This means that the tendons are restricted and the fingers are harder to move.
You mentioned economy of motion being different, but economy of motion simply refers to the amount of movement used. Generally, you should be using the smallest motion necessary to produce the desired sound (obviously whilst still being relaxed). The positions of good technique help you to keep your motions small and relaxed.
This leads us nicely into anchoring, anchoring restricts the movement of the right hand and provides a pivot point. Whilst this may seem to be helpful due to the restricted movement making it easier to perform smaller motions, it is not. The restriction of movement means that your hand muscles are essentially working against each other, one muscle is trying to hold your hand in one place and another is trying to move it elsewhere and tension is a result. This tension will inevitably slow you down; the problem with anchoring is that you are not relaxed.
When anchoring your hands overall positioning will change when you change strings (when picking the high E it may be scrunched up and when picking the low E stretched out). This causes yet more tension and also means you'll be picking differently depending on the string you're on which is again, detrimental to accuracy.
Like I said, it is possible to get good with poor technique and many guitarists do. It will however, take far more work, the results will generally be poorer and you might end up injuring yourself (tendonitis and the like).
