"Here be Dragons" is a metaphor used on maps of old to show places where the known merges into the unknown and dangerous.
In standards, SHALL and REQUIRED, or SHALL NOT and PROHIBITED are well known territory. These are places we have trod before, and know it well. And know whether it is safe or not.
SHOULD and RECOMMENDED, or SHOULD NOT and NOT RECOMMENDED define areas where our knowledge is not so absolute. These are areas where care must be taken. I tell engineers: treat every SHOULD and SHOULD NOT as if the word SHOULD was SHALL. And only when you are expert enough to explain why this conformance statement cannot be SHALL can you tell me that you won't do it. It rarely works, but is good advice. The point is, experts have already gone over this, and unless you understand it better than they do, work with their recommendation.
Where the real dragons are is in MAY, or PERMITTED content. In this, danger abounds. Because it says MAY, people are free to ignore it. The MAYs in a specification are the places where you can do cool stuff, and where new capabilities can be explored. And so, just as danger abounds, so also does opportunity. After all, we all know what dragons covet.
P.S. In the map above, look to the mid-right, in the south-east part of Africa. You should find a dragon, and asp and a basilisk.