Honestly, with all the priority like Talonflame flying around I wouldn't say it's entirely free. Plus, the opponent can always set up at the same time.
Speaking of Talonflame, it can do the same... Protect + Swords dance, now you have +2 180 BP Brave bird Sharp Beak boosted to handle...
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 252 HP / 252+ Def Skarmory: 160-189 (47.9 - 56.5%) -- 34% chance to 2HKO after Leftovers recovery
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 0 HP / 4 Def Tyranitar: 258-305 (75.6 - 89.4%) -- guaranteed 2HKO
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 252 HP / 88+ Def Ferrothorn: 378-445 (107.3 - 126.4%) -- guaranteed OHKO
It still outpriorities almost anything and kills it.
+2 252+ Atk Sharp Beak Talonflame Brave Bird vs. 0 HP / 4 Def Thundurus: 375-442 (125.4 - 147.8%) -- guaranteed OHKO
It's almost as if it would have Contrary, every second turn you get a free +2 Boost so it breaks anything... Use sub instead actually...
Final edit; run it with Acrobatics without an item instead:
+2 252+ Atk Talonflame Brave Bird vs. 252 HP / 252+ Def Skarmory: 174-205 (52 - 61.3%) -- 97.7% chance to 2HKO after Leftovers recovery
+2 252+ Atk Talonflame Brave Bird vs. 252 HP / 16+ Def
Mega Aggron: 132-156 (38.3 - 45.3%) -- guaranteed
3HKO
+2 252+ Atk Talonflame Brave Bird vs. 252 HP / 0 Def Regirock: 165-195 (45.3 - 53.5%) -- 2.3% chance to 2HKO after Leftovers recovery