Negative priority resources should not be considered when delivering messages without a resource.
This is my interpretation of section 11.1, paragraph 4.1 (http://www.xmpp.org/rfcs/rfc3921.html#rules):
However, the server MUST NOT deliver the stanza to an available resource with a negative priority; if the only available resource has a negative priority, the server SHOULD handle the message as if there were no available resources (defined below)
The part defined below is paragraph 5.3:
For message stanzas, the server MAY choose to store the stanza on behalf of the user and deliver it when the user next becomes available, or forward the message to the user via some other means (e.g., to the user’'s email account). However, if offline message storage or message forwarding is not enabled, the server MUST return to the sender a stanza error. (Note: Offline message storage and message forwarding are not defined in XMPP, since they are strictly a matter of implementation and service provisioning.)
I’'ve tested with 3.2.2 and if I only have negative priority resources connected and I send a message to the bare resource, the message should be sent to offline storage.
I think this is a bug, and it prevents the usage of non-chat agents with Wildfire.