hydro on a warrior is good for armour of frost, and having a second character with restoration (not just for the heal but to get rid of negative statuses) is never bad
putting points in hydro on a Str build means not putting those points in damage dealing i.e. Warfare/Scoundrel/Geo/1H/2H etc. It's interesting that many players are entrenched in this mentality that the best way to restore HP or armor is by using spells, even though there are more efficient ways to restore those. Hydro certainly has its uses on other builds that specialize in restoration/hydro damage, but not so much on a DPS warrior.
Again, using Five-Star Diner + Living Armor, a single dinner restores 30% of Vitality AND about 30% of magic armor. A single Large Magic armor potion with Five-Star Diner restores more than Armor of Frost! Plus you can eat/drink 4 items every single turn, no cooldowns!. And if you still want to use Armor of Frost, you can easily make the scrolls, which also dont have cooldowns AND dont' need points in Hydro.
So, spending points in Hydro just to be able to cast Armor of Frost ONCE every 4 turns, when you can restore many times more armor with consumables every single turn, is a complete waste.
