I think it depends on what your trying to teach, or achieve personally as a fighter.

Successful fighters do not use, or maybe even know the fast majority of things they might, if they had taken the grading route. (the same is true for people who grade only, will never ever gain the experience you receive as a figher)

Why? Because for them, what they need in the ring has to be effective!

The bread and butter 1,2, roundhouse kick has no relation to say spinning back fist into a flying knee.

All techniques Muay Thai, but effectiveness of the combination can easily be seen.

Here is where the grading steps in, for my fighters and probably for most instructors, we narrow down the MT arsenal to what we decide is very effective and worth training hard on.

But when it come to the gradings, we are allowed to explore Muay Thai to its fullest, and include so much that we know, may not be that effective in the ring, or legal in the ring, or just no bloody use to anyone.