If there was a "smiley face" for "Standing Ovation" - I would use it here!
You have concisely captured a very valuable perspective, not just for "GTD" but for other aspects of life as well.
There are definitely some "Universal Truths" that run through everything - whether it is Time/Life Management or Self-Help. However - the path that one chooses to discover them is uniquely one's own - and we should each respect each others' paths.
There is no "one road to Rome," and those that cling to anything in that manner, with an almost religious fanaticism, are simply limiting their own experiences through narrow (but simple to handle) beliefs.
Each of us are "students" in some capacity or another, and we have to find the teacher, or combination of teachers, that is appropriate for ourselves.