Two handed sword 18 str 14 dex
AC from Parry and related feats = +6
Steel Greatsword 1-12 +1-6 (steel) +6 (1.5x str bonus) = 8-24, avg 16
4 attacks per round = avg 64 dmg per round
Bastard sword 18 str 16 dex, nothing in off hand
AC from Parry and related feats = +9 Winner: Defense category
Steel Bastard Sword 1-10 +1-6 (steel) +4 (str bonus) = 6-20, avg 13
4 Attacks per round = 52 dmg per round
Bastard sword 18 str 15 dex, shield in off hand
AC from Parry and shield and related feats +8 (the way the skill check penalties work, you lose 1 parry ac for heavy and 2 for tower, so you can only ever get total +8 no matter what size the shield is)
Steel Bastard Sword 1-10 +1-6 (steel) +4 (str bonus) = 6-20, avg 13
4 Attacks per round = 52 dmg per round
Bastard Sword and Short Sword 18 str 16 dex
AC from Parry and related feats = +8
Steel Bastard Sword 1-10 +1-6 (steel) +4 (str bonus) = 6-20, avg 13
main hand 4 attacks per round = 52 dmg
Steel Short Sword 1-6 +2 (steel) +2 (0.5 str bonus) = 5-10, avg 7.5
off hand 2 attacks per round = 15
avg 67 dmg per round Winner: Offense category
So, the only way someone with a shield can hope to get close to the same defense as someone with nothing in their off hand, or a dagger in the off hand, is to pump way too much points into dex and/or enchant the shield, losing a level to boost an item that they might drop and loose if they get killed.
This is not ok. This is broken.
Three things need to happen to fix this:
1) Remove the 15 dex requirement from Shield Parry. You don't have to be a ballet dancer to intercept a blade with a shield. That's why shields exist. A shield is a pretty big target, it's hard for your opponent not to hit it. That's the point of a shield: your opponent hits it instead of you.
2) Have Shield Parry feat reduce the parry penalty for using a shield by 3 (by granting +3 parry when a shield is equipped). This would make shields as follows when the character has Shield Parry feat: Light = no parry penalty, Heavy = -5 parry, Tower -10 parry. Remember, all a two-handed sword user has to do to get full parry bonus is get Improved Parry, which grants +4 parry.
3) Make Bonetti's Defense (love the name) work ONLY with light, finessable weapons. No one's fencing with a 10 pound bastard sword. I don't care how strong you are, wrists can't move something that bulky that fast.