So after a number of hours of playing, I've noticed this perk being used by almost every player, and I've noticed it having a huge impact on gameplay.
People are mostly using it because it's the earliest perk you get in t2, but I think most people are realizing that you can't effectively fight someone who has Toughness, unless you also have Toughness. It's like being in a fist fight with someone where they can't feel pain and can't flinch or be staggered.
In every other CoD I'm used to shooting someone first and having that advantage of their recoil and inability to react. In Black Ops 2, it's a matter of which person's gun has a faster time to kill. It's become impossible to fight more than 1 person since you can't cause them to miss by shooting them, so two people with mediocre aim will kill you even if you have perfect aim.
Quickscoping has become impossible to deal with when the sniper is at least decent. If you shoot them 2-3 times from a distance, it doesn't matter as their aim is not affected at all. They can simply stand there and take their time; a quickscoper needs less than a second to kill you, and Toughness means that you can't stop them.
Fighting submachineguns with assault rifles or LMGs (or vice versa) has become almost impossible as well. At close range, the submachineguns have a faster time to kill, and at long range the assault rifles and LMGs have a shorter time to kill. In past CoDs it's been possible to fight at close range with long-range weapons, or fight at long range with close range weapons. In Black Ops 2, you can't force the enemy to flinch. All they have to do is aim and fire when their weapon has the range advantage, and there's nothing you can do about it.
I've resorted to using Toughness as well since the alternative is to constantly lose. However. it's still frustrating as I explained above.