Alice is at work, and messages Bob who is using Pidgin. Bob sees a message from <xmpp:firstname.lastname@example.org/work>.
Alice starts to leave, but realizes that she forgot to ask Bob what his plans were for that night. She sends a message to Bob from <xmpp:email@example.com/mobile>. This message appears in the same window as her previous messages in Bob's Pidgin client. Bob responds, but his messages continue to go to <xmpp:firstname.lastname@example.org/work>, even if its status is "away" and its priority has dropped below that of /mobile.
Alice never hears back from Bob, and they don't see each other that night. Alice does go out, meets a cute guy, and lives happily ever after. Bob ends-up complaining about missing configuration options each night for the next year since he has free time because he blew it with Alice.
OK, this is working pretty well. Time to upgrade for no reason.