Yes, if we know a reason why you might want to vary from what we recommend,
it's
always a good idea to explain that. But that's a side issue, and it doesn't
really matter. The fact that we can't think of why you might want to...
doesn't
mean that we should forbid it.
I guess the whole question is: Is it important to us to keep the
subject in case no other subject has been specified?
Suppose (I'm groping, here, but bear with me) someone implements Sieve
notifications and sells it to a hospital.
[privacy issue with passing subjects]
If privacy matters, then don't pass subjects to pagers.
Arnt pointed out that clients may be buggy. And we all know, some have
a security problem with malware attachments. If that means an RFC is
well advised to say you SHOULD NOT alter the content in transport, we
SHOULD keep the subject. If the RFC is fine to ask you MUST NOT alter it,
knowing some sites do nevertheless to let their broken clients be safe,
we can do as well.
Perhaps this analogy helps to decide.
Michael