WoW Anniversary Buff after WoW Birthday event [9 % more xp /rep]

I just noticed something very cool on my monk ....If you haven't opened your letter with the WoW Birthday item inside yet ( for example if its just some little twink) you can do it now, and the item wont disappear , so you can use the buff.

Well its kind of useful, because you will get 9% xp/rep more.....

Unfortunalety it disappears after you relog, but if you don't logout after opening the letter, the buff wont go away and you can simply refresh it.