The ideals of IAM are fine.
I’d definitely prefer that the driving public gave some thought to their driving/riding and the impact of their actions on others.
If everybody undertook the IAM course then that would be no bad thing. Observation and planning is is a good thing.
Being a member shouldn’t provide a sense of superiority though.
Theirs (and Rospa’s) system is the slightly quirky, Roadcraft method of driving, but it is not “the one, true way” to smooth car control.
I joined and passed their test(motorbike). I’m grateful to the volunteers who run it, but I’ve no interest in being a member of the club. I didn’t re-new my membership.