Can you start actually reading what you quote? "The implications must be understood and weighed" does not mean "you are required to have a good reason not to do it". I can know of the Message-ID field, understand what it's used for, and carefully weigh the possible lack of interoperability against the effort required, concluding that I'm too lazy to do it right now. I have then understood and carefully weighed the implications before choosing a different course, but I don't have a good reason to.

Words mean things, especially in standard texts. You can't just carelessly rewrite sentences using not-quite-synonyms like you're doing here.

Oh, and that "must" is not a "MUST". Had the text said "The full implications MUST be understood ..." it would have been a proper requirement by the standard, but this lower-case "must" is just normal part of prose and not the magical "MUST" word which formally imposes a requirement.