The RFC 2119 imperatives MUST, MUST NOT, MAY, and SHOULD are used extensively in the format and protocol drafts to describe XML content models, which is unnecessary.
Open -- Editorial.
The XML element content models within the Atom Internet Drafts are part of the "basic specification" of the Atom format and protocol, like the use of ABNF (RFC 2234) in traditional RFCs. It is unnecessary to use requirements imperatives for the cardinality or markup syntax except "where it is actually required for interoperation" (RFC 2119, Sec. 6).
Example XML-based RFCs for comparison:
RFC 3017 -- XML DTD for Roaming Access Phone Book. M. Riegel, G. Zorn. December 2000.
Sections 6.1 and 6.2. Uses DTD syntax to describe content model.
RFC 3275 -- (Extensible Markup Language) XML-Signature Syntax and Processing. D. Eastlake 3rd, J. Reagle, D. Solo. March 2002.
Section 4. Uses XML Schema to describe content model.
RFC 2518 -- HTTP Extensions for Distributed Authoring -- WEBDAV. Y. Goland, E. Whitehead, A. Faizi, S. Carter, D. Jensen. February 1999.
Section 12. Uses DTD syntax.
First pass: de-emphasize RFC2119 imperatives where used for content models or markup syntax.
Second pass: rephrase for easier reading using a less formal tone.