Regarding Rend and TFB, unless I missed something, an interesting point would also be "how long the other targets are going to last" because nowadays, spending any Rage on Rend on fights like Galakras (biggest AoEfest there is, with Garrosh) is a complete waste of rage, adds die in 3-4 GCDs, so if you already take those GCDs to apply Rend to 3 of them, you did litterally no damage and got maybe your initial Rend cost in rage back from TfB.
At the moment, the only fights where I bother applying Rend are Boss fights, and on the boss only, everything else dies way too quickly. And I think that highly devaluates TfB's appeal ...