I agree with your assessment of Scott Myers' book and approach. I think it's a foundational step for beginning writers.
This is my general take about the bulk of the advice factory.
* Most screenwriters aren't very good. (Even the best writers aren't infallible, hence Goldman's statement about nobody knowing anything.) 99.99% of scripts out there wouldn't make very good films even if given decent production / direction. (Occasionally a good story gets screwed over in the development process, but I think that problem is probably much rarer than most people think. Most mediocre movies are the result of mediocre storylines.)
* Any writer following the advice from the factory will probably be able to turn a bad (i.e. uninvolving) script into a mediocre script (making the first or second cut in a mediocre competition, e.g.). How-to books by Snyder, McKee, Truby, yadda yadda are essentially about polishing a turd. The final result is shinier but still a turd.
* The advice factory offers rational (left-brain) advice, but most writers using the advice factory are essentially already rational / left-brain writers. Writers who craft engaging stories typically do so by relying on their intuitive (right-brain) skills (they watch a lot of movies and read a lot of scripts, rather than reading a lot of how-to books). The weakness of most how-to books is that they don't engage the intuitive side of their readership (even fuzzy, abstract writers like McKee).
* Promotional blurbs for how-to books typically cite 5-20 writers who state how reading that book helped them write / sell / a particular script. What the blurbs don't tell you is how many thousands (probably tens of thousands) of writers followed the advice of the guru to the letter and still ended up at the bottom of the slush pile.
* There is a place for analytical (left-brain) writing strategies. But if that approach substitutes for intuitive writing (much as junk food substitutes for nutritious eating), then it is an obstacle, not a help. Still, after eating a one-pound bag of potato chips chased with a few bottles of Mountain Dew, you will feel full, just as if following the advice of Blake Snyder to the letter, you will think that you have crafted an engaging story.
