— Section 3 —

   Once the UIDONLY extension is enabled (see Section 3.1), the client
   MUST NOT use message sequence numbers (including the special marker
   "*") in any arguments to IMAP commands, and the server MUST return
   tagged BAD response if the client uses message sequence numbers.  The
   server SHOULD include the UIDREQUIRED response code in such BAD
   responses (see below).

I see nothing below that says why this is SHOULD rather than MUST, and I can’t imagine any justification for not including it.  Why is this SHOULD?