It seems to me that there are two important cases here:
* The MTAs of concern are under the control of either the sender
or the receiver. In this case, the "guessing game" is no worse
than the one that is played with every message: you need to
guess what "from:" and "subject" lines, and envelope address to
use to maximize the chance that I will read your mail or, put
differently, which ones to avoid to minimize the chance that I
will delete it without reading (or have agents or filters do
that for me).
I guess I disagree - first of all, we don't usually expect to
guess "From". We do have to choose a subject, but that choice
is informed by our vast experience in communicating between
human beings with words. Priority is less obvious, and it
adds another dimension. And if we're just making a facility
for filtering by the sender or receiver, an Importance header
works just as well (are we really going to set this separately
on a per-recipient basis?)
* The MTAs making the decision are "in the middle", and not
under the control of either sender or recipient, or anyone both
accept as an appropriate intervening authority. That scenario
is basically equivalent to all of the other interception objects
on the network (what you think about those is well-established;
let's not have the discussion here again), but with one major
difference -- there will be no intermediate MTAs doing stupid
things out of ignorance of the protocol, since a priority
parameter cannot be sent to any MTA that does not advertise it.
That is a small advantage for this case, although perhaps a
trivially small one.
I guess I don't consider things to be interception objects unless
they actually intercept traffic, and an MTA doesn't qualify.
My question is - why is it useful for Internet mail users to
allow/encourage intermediate MTAs to do arbitrary filtering
based on an unauthenticated priority?
In other words - I do think a priority facility could be useful,
but I think we're missing some significant pieces from the current
design that would be required to make it useful. And I'm not
quite sure what pieces are missing, or whether it's feasible
to supply them.