Most people are saying don't use it, a couple others are saying the others are confused and we should use it(though haniblecter only has a lvl 30 War based on his profile in this thread and keeping hate at that lvl is actually quite easy so it is probably working fine for him)
I currently have a lvl 48 Warrior and subbing Monk and am the main tank in my set party(yes I know my sub is my first problem but that is a different discussion - currently I am working on my Ninja sub). I have a thief in the party which helps me with initial hate control. After that it is up to me to try and maintain it which I do a very good job of.
My experience with Defender in the 40 levels(more so after 45) is that using defender doesn't make up for the loss in attack. You may get some hate from using it and a little more hate from the lowered attack of the enemy on yourself. (though after losing hate then using defender, it has never brought the hate back on me).
It does help my defense by taking it from 300 up to about 361 with Protect III and a Fish Mithkabob. My Vitality is also at 55+16.
I end up losing hate quicker with it on than with it off. My attack drops enough that I can go from hitting bats in Garlaige from 60-100/hit all the way as low as 0-40 on IT mobs.
Yes the mage does have to heal a bit less, but with a good Red Mage(with refresh and convert) and White mage, down time is minimal at the worst of times.
My experience is don't use it unless it is an emergency (i.e. down in the red for hp and no cure coming your way).
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48War, 25Mnk, 21Rng, 22Thf, 12Nin, 12WHM