I one group Halls of Testing quite frequently w/ 3 people who each 2box one additional char (war.dru,clr,shm.sk,ench,all 65).
We also regularly do Zlandicar,Sev,Talendor, and several other dragons/raid oriented type mobs and w/ 4 caster/priests in one group you pay attention to AoE's, and in most cases resist gear isnt worth it, and resist buffs seem to cut it just fine.
As far as resisting the AoE's level has much more of a factor than resists.
I have played both a caster and a melee in many situations where AoE mobs and their dynamics come into play and I have never once bought or needed resist gear. Unless you can get your resists to 250+ its not even worth it to lose HP for minimal resists.
In the case of HoT, casters can always hide behind the AoE wall. Our Shaman always just chills right next to the mob, AoE or not. Get the levels first, then you will notice more resists, resist gear in a place like this is futile and uneeded.