DEX is not useless from a tanking standpoint, it's just not worth taking instead of vitality. When you can get it in addition to vitality it's nice. It's much better for Paladin than it is for Warrior though, because it also increases block. On a Paladin 15 dex = roughly 2% mitigation. Parry has more raw contribution to parry than DEX though, so if you want to parry more often, you just want raw parry.

For a Paladin though, as of right now DEX > Parry, this is why crafted accessories like gryphonskin are much better than rose gold accessories or darklight, because they have an entire extra stat because the itemization is made to be useful for both bards and dragoons/monks, meaning it has STR, DEX, 2 secondary stats, and can still be slotted with as much VIT as darklight/rose gold.