I was just thinking - Guard redirects 50% of incoming damage at a target to you, meaning you could always cut the damage incoming on a tank by half easily. Do you think end game content will be designed around the idea that you will be using Guard on the other tank always? There seems to be some fun strategy behind it's use in PvP, but it seems like the best course of action in PvE would be just to have it up always - if you're the off-tank and not currently taking any damage, it's all good, and even if both of you are tanking damage, it's still a gain because you have 2 chances to avoid some damage - the first player might avoid some, and if he doesn't, 50% will go to the second player, and he has a chance to avoid some too, so it's still a gain overall.
Another thing, if Player A has guard on Player B and vice versa, and Player A takes 100 Damage. 50 of that goes to Player B through Guard, and 50 stays on Player A. Though with Guard from Player B being on Player A, would 25 of that 50 then also transfer back to Player A, and then 12.5 back again, and so forth? I hope you understand my question here, if I am taking 50% of the damage the other guy takes, and he is taking 50% of what I take, then is there a never ending loop of us each taking 50% of a hit that the other took back and forth?