Hey its fun an all that you are using things like level 6, Power Attack & THF vs Power Attack and TWF on the same 6th level fighter without any real items. But when you start branching out beyond the same character with the same stats with the same class you start noticing things.
A 10th Rogue with 18 str using Power Attack(5) and using a +2 Longspear.
+1 Longspear +3/-2 melee (1d8+16+5d6 & 2 str dmg), assuming all hit (yeah right) thats 76 damage & 4 str dmg.
OR
A 10th level Rogue with 20 dex, 14 str, Weapon Finesse, ITWF, and using two +1 Daggers
+1 Dagger +10/+10/+5/+5 melee (1d4+3+5d6 & 2 str dmg), assuming all hit (every one of them has a better chance than THF), thats 92 damage & 8 str dmg.
Now factor Craven, 96 vs 132. Wounding & factoring con loss? 106 vs 152. SA boosting items? Gap keeps widening.
Now what if we skip full attack and focused on charging?
A 10th level Monk, 18 str, Power Attack(7), Leap Attack, Mantis Leap, Shock Trooper, Battle Jump. With a well placed move action.
+1 Valorous Staff +12 melee (3d6+109). Monks are the powerful, far beating the rogue using THF or TWF.
Lesson learned? Yeah I hope so.
Got bonus damage? TWF.
Don't? THF.
What about S&B? Use a buckler, -1 to attacks for all the benefits of using a shield at -1 AC. And later on when miss chances are better than AC, you won't feel bad for using a +3 buckler to hold a greater shield crystal.
What if you really get down to it? You can deal a crap ton of overkill damage using any style. So I suppose the extra AC is useful, but then you pick up more disliked for their unbalanced traits DMs don't like.