I assume you are talking about the Legionnaire here and failing to consider the Reaver, Katari and Templar. These classes have nowhere near as much survivability as the legionnaire and even then I would disagree that it's practically invincible. Especially when taking 2x, 3x or even 4x damage if you used the shield and one-handed weapon combo. I'm also assuming that you are talking about a high level character and not characters that may be low or low to mid level, these again are far from invincible for the majority of players. Unless of course they are playing on a difficulty which is easy for them.
I stand by what I said and that I really see no use in this perk on a weapon or shield. Give me a weapon / shielf with 5% att for each enemy within 8 metres any day of the week, practically all the benefits of the items with berserk (sometimes more) on them and none of the penalty.
Yes I use a lvl 20 legionaire and the 20% damage boost is extremely good. 200% or 2x dmg with unlimited guard and invincibility is exceptional when there is no downside. Also a cooldown reduction amulet is amazing on legionaire.
Payback Strike
War Cry (guard generator & aoe aggro)
Walking Fortress (guard generator and invincibility)
Counterstrike (guard generator)
Use War Cry for aggro then Walking Fortress which will make you invincible and put you at full guard. Then use the Payback Strike once WF is over to knock down everyone around you and give your guard some extra uptime. Once your guard drops after PS use your Countrestrike to put your guard back at full. All cooldowns should be close to reset by now so rinse and repeat this process while tanking and you will not die, even with the extra damage you're taking. I barely even notice I'm taking a bit more damage than normal because of how tanky the class is.
I haven't played much Templar but I'm sure you can make an effective tanky build using this type of item as well.