I fully agree with this (from #4), including the part about BE texts. Sadly, I "get" to edit/proofread texts that are for the most part written by BE speakers. I'm a bit less strict when it comes to applying the standards adopted by pertinent style guides when the target is BE, but I show no mercy ;-) when it comes to texts that are supposed to be AE.
Well, if something is unanimously recommended by style guides (which this is), it seems to me that that's as close as we get to a hard rule in English.
It's true that especially British texts often ignore this particular rule, as with certain other rules about punctuation, so no one should be surprised to see the hyphen mistakenly omitted, time and again, in, say, the Guardian. But AE editors and writers are generally more careful, and people learning English should follow the rules, not ignore them.