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Managing guard isn't that very easy and it requires some decent coordination from two players. It's not exactly a terribly complex thing, but still few tank-guardee pairs do it well enough.
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Or just, well, lots of raw damage that even with the guard, will destroy their guardee. One thing you can hear very often as counter to burst damage is: "have a tank guard you (and have healers)". The ability is a pretty core element of the game as it is.
But in the end this is mostly about what the role of a tank is. The two main flavors being DPS tank with twohanded and defensive tank with shield. The DPS tank can guard, but since they have no shield and likely less endurance so they can actually deal damage, they will take more guard damage and this also has a greater impact since they don't shrug off damage directed at them that easily either. Guard is entirely fine as it is for the DPS tank.
And then there is the defensive tank, whose main, if not sole, justification of existance is exactly the powerful Guard ability. It's true that they do have knockbacks/knockdowns, snares and a variety of buffs/debuffs, but so do DPS/healer classes, so that's likely not enough to take a deftank instead of a DPS or even a DPS tank. How many times has it been said, in the game, forums and generally everywhere, including frustrated players shouting at their team/scenario-mates, that one should attack everyone but the tank? Even with all the utilities they have available, a deftank is a nuisance at best, not a threat. The only reason people ever notice them is, yes, because they are guarding someone and doing it actually well enough to make a difference (which, again, doesn't depend only on them but the person they're guarding as well). Take away that ability, or even make it not so very powerful, and the defensive tanks will start to turn into another flavor of DPS.
So we have a strong ability that can become the center of the battle and dictate group tactics - if used well. There is definitely a good amount of the infamous "l2p" here, as the player skill of the two opposing parties will make or break how much a difference a guarded player (in most cases a melee DPS) will mean. And it's pretty much everything a defensive tank can contribute to group play (everything that other classes couldn't do, possibly better, that is).
But... if the Guard ability must absolutely be nerfed, this seems the best way to go about it.
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