For me, the requirement for NSID comes from the requirement for a resolver operator to be able to debug their interaction with the RSS, which is very hard without a unique identifier being returned within the NSID field from that operator.  Thus a requirement to make the system more fail-safe/debuggable.

I do get your point about the split, and applaud it.  For me, the RFC should contain technical elements and the RSSAC001 document should contain policy elements.  I think the point we're stuck at is "should there be a place to document the 'we want to have X, but it's not a requirement' and if there should be a place/list, where should that place/list go?".  Certainly one alternative is to refuse to have a list, or anything related to a SHOULD/MAY type specification leaving only the "if you don't do this, the system won't work at all".  But then, it gets even more tricky; EG, is Ipv4 really required by *every* RSO?  Are checksums *really* required?  And why MUST they be validated?  Does the system as a whole fail when these don't apply?  The document could be very very short if we only stick to the "do this or it breaks" list.  But, you're right it's a slippery slope and somewhere a line needs to be drawn or at least organized into multiple lines.