It is my understanding that when the warrior is not at the top of the agro list, taunt will do something like 100 or 200 hate points, and when the warrior actually does have agro using taunt generates significantly less hate. I dont know the exact numbers. But to answer your question, I think it always works in that it generates a certain amount of hate, but it is not always enough to regain agro. So basically it "works" when the warrior taunts a mob and it switches from its previous target to the warrior, and doesnt work when the warrior does not gain agro. And if you are asking to know whether or not it works when you already have agro, I think it always does work, but its not really a means of sustaining agro as much as regaining it. Maybe I am wrong, but if Im correct a message would be useless.