The justification for MUST was I suspect to prevent infinite recursion mail
loops. For that particular purpose a count is sufficient.
Oh, of course, you're right, it even says so in section 6.2 of RFC
2821, now 6.3 in RFC 5321. But there isn't a count, so this would be
an incompatible change to the SMTP protocol, which seems like a
non-starter to me.
R's,
John