It says "SHOULD" not "MUST", so the client can assume the server will keep its promises, it's just not recommended.
"SHOULD This word, or the adjective "RECOMMENDED", mean that there
may exist valid reasons in particular circumstances to ignore a
particular item, but the full implications must be understood and
carefully weighed before choosing a different course."
http://www.ietf.org/rfc/rfc2119.txt