Resistance does not work against Bosses AOE dot attacks. It does work against Defilers ToS, however, it is different. Instead of it reducing damage, it reduces the chance the DoT will be successful. Keep 2 sets, one enchanted against casters and another against melees (which includes most mobs). There have huge Requiem Lant in our site.
Beyond a certain point, defense is useless which happens before you reach a point where you cannot get a considerable amount of resistance to 1 element. We think the harder question is if you should spec yourself with Damage and Crit xeons instead of defensive xeons. The level 65 PVE obtained sets come with an element resistance which helps achieve high resistance. The level Requiem gold 65 PVP obtained sets have damage increases.
Getting 2 sets of gear in Requiem is impractical and time consuming. Only the millionaires can make this a possibility IMO. We would suggest if you are melee to get resistance gear, if you are a caster get damage gear. Casters have a far less chance of being targeted then melee do. Melee usually has a problem reaching a target as opposed to dealing enough damage.