barrier tusk is terrible! I use my 509 enhancing scholar alt to apply phanlx and I receive 0 dmg taken from a majority of the monsters in ffxi, If I try to use barrier tusk even with -50% pdt armor on if u add it up its 65% so an attack for 100 should hit me for 35 dmg where as if I use diablos phalanx, or a scholar/rdm phalanx it is significantly higher because it is "-damage taken" barrier tusk is just for physical damage and it does not stack with any other form of phanlx, blu/rdm's phanlx is waay better (more so since the x2 enhancing swords were released) so I literally cant find any practical uses for barrier tusk.

and to clear up this whole "it breaks the cap" NO it does not (breaking the cap would insinuate that I would be allowed to equip more pdt armor)

think of it like you would haste, you can ONLY HAVE A MAXIMUM OF 50% physical damage taken from armor, after that you can use other sources until you reach the cap of -85% damage taken. Example 50% physical damage reduction (capped gear) now I'm going to have to find other forms of - physical damage taken to reach the 85% cap (cant remember if its 80 or 85)