Outspeeding every single Choice Scarf user and OHKOing all of them with the exception of Choice Scarf Jirachi is huge. Tell me, which other Swords Dance user has that advantage? Furthermore, Talonflame can use an Adamant nature to have the equivalent of 92 Base Attack (assuming its Base Attack is 80) and its main STAB has 110 BP with terrific neutral coverage. Flame Charge or Flare Blitz covers any Steel-type (all of which are slower, except from Choice Scarf Jirachi), and you can even use Roost with some HP investment if you want. For example, with Roost, the standard Choice Scarf Jirachi has nothing on you, as you can just not give a shit about its usual attacks (Iron Head, Ice Punch, U-turn) and just 2HKO it with Acrobatics if it goes for Trick. Furthermore, priority Acrobatics makes this Pokemon basically immune to most priority, as the most popular priority users in OU are Breloom, Scizor, and Mamoswine, all of which are boned by a +2 Acrobatics and are outsped. Only Extremspeed users can hit Talonflame before it uses Acrobatics, and even then Talonflame can take the hits rather well if it opts to invest in HP (plus Roost).
So, you have a Swords Dance sweeper immune to Choice Scarfers (bar Choice Scarf Jirachi), immune to priority (bad Extremspeed), and with awesome STABs, that only really minds physical walls not weak to its STABs and some Rock-types (the only move it has to hit Rock-types that resist Acrobatics is Steel Wing so far). And you compare this thing to Crobat? Its only real disadvantage is its huge SR weakness, but we all know that this doesn't prevent a sweeper from succeeding in OU if its worth it (Volcarona).
Oh, i forgot to mention what an amazing revenge killing tool priority Acrobatics is too. No more getting swept by QD Volcarona, Chlorophyll Venusaur, or Choice Scarf Keldeo, Talonflame is here!!!
Also, some calcs against 252 HP / 252 Def+ Mega Aggron for fun:
- +2 252+ Atk Landorus-T Earthquake vs. 252 HP / 252+ Def Filter Aggron: 225-265 (65.4 - 77.03%) -- guaranteed 2HKO
- +2 252 Atk Terrakion Close Combat vs. 252 HP / 252+ Def Filter Aggron: 226-267 (65.69 - 77.61%) -- guaranteed 2HKO
- +2 252+ Atk Life Orb Lucario Close Combat vs. 252 HP / 252+ Def Filter Aggron: 287-339 (83.43 - 98.54%) -- guaranteed 2HKO
- 252+ Atk Choice Band Scizor Superpower vs. 252 HP / 252+ Def Filter Aggron: 126-148 (36.62 - 43.02%) -- guaranteed 3HKO
- +2 252+ Atk Scizor Superpower vs. 252 HP / 252+ Def Filter Aggron: 166-196 (48.25 - 56.97%) -- 90.23% chance to 2HKO
- 252+ Atk Choice Band Tyranitar Superpower vs. 252 HP / 252+ Def Filter Aggron: 127-151 (36.91 - 43.89%) -- guaranteed 3HKO
We just might have a very decent alternative to offensive pivot Landorus-T, as a set with SR, Steel STAB, Ice Punch, and Fire Punch can deal with many physical attackers. However, lack of any kind of recovery and mediocre special bulk are its weak points.