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.