Maybe you have notice that vs mobs it seems to make 0 difference in how hard or how often they hit you. if the mob is red or white they do the same dmg to me no matter what i do buffed or not buffed coudl even take half my armor off and it doesnt change. For instance you ever notice how when a red mob hits you you get knocked around in Ragnarok online, well the higher defense you have the faster you can recover from hits or be totally unphased by them completely. For instance level 54 mage with very high defense (over 650 Ragnarok online zeny) can lead an aoe train alot better then some knights who totally neglect defense.
For PVP defense is extremely I amportant because when someone higher level(2moons powerleveling) hits you if you stumble it will interupt your skills. The higher defense the less you become interupted. But as far as damage it has no effect to Ragnarok zeny. Damage is based on level vs level only, mitigation is only achieved from physical resistance, and avoidance from Gaurd and block. Exorcists are pretty much staple mercenaries for just about every build in the world. Its really hard to write anything more in a breakdown for the Exorcist because he is really necessary. You cannot think of a single build that would remain competitively effective at higher levels with no Exorcist presence, simply because no Exorcist presence in your enemys formation would simply not force you to push forward your standard strategy. Exorcists are something you have to work around, and having that kind of clout in a mercenary is just especially beneficial.