I'm not arguing that this is wrong, though it sits wrong with me.
From the Main FAQ:
Do you have to actively wield a weapon of defending to
use its power? Or could you hold a longsword of defending
in your left hand, not use it to attack (so you are not
actually using the two weapons) and still wield a sword in
your right hand without penalties? Or is the defending
bonus considered part of the normal parrying that happens
in the background?
Using a weapon of defending works just like the Expertise
feat. (You have to use an attack or full attack action.) You can’t
use the weapon like a shield; if you hold the weapon in your off
hand and claim an Armor Class bonus for it, you take all the
penalties for fighting with two weapons, even if you don’t
actually attack with the weapon.
I'd extrapolate from that that making a Full Attack Action that utilises more than one weapon held simultaneously - whether you actually claim the extra attack or not - incurs TWF penalties. Call it a coordination thing.
Now, it gets murkier when we start playing with the possibilities...
Assume I have +6/+1 iterative attacks and the Ambidexterity feat.
If I attack with the longsword in my right hand, drop it, quickdraw a shortsword in my right hand, and attack again, do I incur penalties?
If I attack with the longsword in my right hand, drop it, quickdraw a shortsword in my
left hand, and attack again, do I incur penalties? Do I get full or half Str bonus to damage on the second attack?
If I attack with the longsword in my right hand, shift it into my left hand, and attack again, do I incur penalties? Do I get full or half Str bonus to damage on the second attack?
If I attack twice with the longsword in my right hand and then shift it to my left, can I make a third attack (taking TWF penalties on all three) for attacking with two weapons?
If I attack twice with the longsword in my right hand, drop it, and Quickdraw a shortsword with my left, can I make a third attack (taking TWF penalties on all three) for attacking with two weapons?
-Hyp.