Focus fire = beneficial (concept)
Back many months ago, I suggested a non-linear healing or toughness trait improvement that took into account incoming dps. Necromancer got nerfs instead because Dumbfire in a hybrid zerker build did too much damage but the vampiric skills were buffed to compensate.
I equip Plague if i know that i will be zerged while trying to get into a tower or keep. Better option than Lich imo.