Priority tag for routing messages with the highest priority?

I notice in the Presence packet, there is a priority attribute for routing messages with the highest priority (Does TIMP have this implementation?). Wonder why this is not included in the Message packet as well?

The priority attribute in the presence packet is a Jabber feature.

Jabber allows you to connect with the same username and the same password several times.

Each sessions of a single user is identified by a resource and a priority:
a "buddy" can address a message to particular resource specifying a to="user@server/resource".

If the message is like "user@server", the server delivers the message to the highest priority resource (==user session).