Toughness

[Sim]

Ran a punch of tests on varying amount of toughness in PvE. Please note that I have 5 Ward Pieces (4 Sent and 1 Bloodlord), so mob damage is 100%.

These are the results.

Test 1: Base
Mob: Warpblade Assassin 41+

Armor: 3806 - 86.5% Reduction
Toughness: 630 - 126.0 DPS Reduction

Auto Attack Damage: 447 (346 mitigated)
Auto Attack Critical: 656 (509 mitigated)
Thrust: 786 (609 mitigated)
Thrust Critical: 1162 (902 mitigated)

Test 2: Increased Toughness
Mob: Warpblade Assassin 41+

Armor: 3806 - 86.5% Reduction
Toughness: 994 - 198.8 Reduction

Auto Attack Damage: 406 (314 mitigated) - 9.2 % Reduction
Auto Attack Critical: 612 (475 mitigated) - 6.7 % Reduction
Thrust: 600 (465 mitigated) - 23.7% Reduction
Thrust Critical: 936 (752 mitigated) 19.4% Reduction

Test 3: Increased Toughness and Armor
Mob: Warpblade Assassin 41+

Armor: 4506 - 102.4% Reduction
Toughness: 994 - 198.8 Reduction

Auto Attack Damage: 348 (372 mitigated) - 22.1% Reduction
Auto Attack Critical: 532 (569 mitigated) - 18.9% Reduction
Thrust: 514 (551 mitigated) - 34.6 % Reduction
Thrust Critical: 720 (771 mitigated) - 38.0 % Reduction

I know I was one of the people against having Toughness in PvE but after these tests I believe that it is worth it. We can see that Toughness effects the ability damage taken from the mobs a lot more then Armor can.

[Rok]

I'm not sure how you reached your conclusion but regardless your data shows a number of interesting things:

Thrust appears to do approximately 1961 base damage with a 4.5 speed and the Assassin has 505 Weapon Skill.
The damage reduction on Auto-attack from toughness is consistant with a 2.0 speed auto-attack (Toughness / 10 x 2), which it likely is.
The damage reduction on Thrust from thoughness is consistant with a 4.5 speed special attack (Toughness / 5 x 4.5), which seems unlikely. It seems more likely that Thrust is an instant cast with a large multiplier which may be due to it's cooldown and the creatures lack of a spammable special attack. It may cast Thrust every 4.5 seconds but have no other specials which justifies it's 4.5 speed multiplier.
The mobs Weapon skill has divided both armor penetration amounts rather than subtracting from them. i.e. 50% armor penetration would take 70% mitigation to 35% (70 x 0.5) rather than 20% (70 - 50).

The conclusions I would draw are:
1) Armor Penetration from Weapon Skill divides mitigation rather than subtracting from it.
2) Weapon Skill debuffs are really good.