I think it's a bit more complex than your scenarios, given that
- without authentication, there's no reason not to ask for the
- consequently there will be some incentive to bias an MTA against
high priority messages, or to ignore priority
- if you do have authentication, the effects of priority are likely
to change from one sender to another.
thus asserting priority can easily give you better treatment
with one MTA (the one which recognizes your authentication credentials)
and worse with the next (which doesn't).
it might not prevent delivery of your message, but it might increase
the variability/predictability with which messages are handled.