Ya I think Frothing Berserker would only work (over Carnage and Massacre) if you opt to not use Carnage at all other than a reset of the buff (which is annoying cause, you have to grind out the full brunt of rage bar again)- and just rotate the other three (BT/FS/RB).
That said, in comparison to Carnage and Massacre, Frothing Berseker (technically) is synergistic with AoE situations due to Wrecking Ball procs and bladestorm (and you can reset the buff with using a non-wrecking ball Whirlwind. But pure AoE situations, I guess, will be reserved to trash packs.