I think the use case for Birstling Fur is when you can use it when it overlaps during a period of many attacks. The biggest example I remember is with Kil’jaeden during Tomb of Sargeras. His tank mechanic was a quick series of 5 attacks that each caused you to take increasingly more physical damage. BF lines up with that mechanic, which allowed us to rapidly fill up our rage bar to dump multiple ironfurs during this mechanic window.