Against resistance drainers,a plate is more helpful that a res mod.
But if you go against builds that don’t break resistance,then yes,it does help.
For example,I have an epic heat res on my primary.While I take a lot less damage from weapons like CL,Savagery,Sorrow,stomps,Murmur and Nemo,that +24 res doesn’t make too much of a difference against DB,SC,Deso,Void etc.
Use epic res only for mass drainers or mass heaters,not against damage-oriented builds (and especially not against phys).
Myth res,on the other hand,is worth it,especially the single res modules.You gain whole turns.