What about against mobs that you can't leech from? Insight is nice but I see those warlocks dying to particularly difficult mobs, cursed cows, heralds with powerful ranged attacks. Miss one ES when you can't afford to, and then that can be all over. Doesn't matter if your Hephasto has 300k life if you are a glass cannon with 4k.
True, thats when lethargy and hex siphon come into play, as with WHOLE LOT OF DMG from Insight. Mobs who you cant leech from are usually super weak, except for souls, that are glass cannons, thats why i talked about that the only resistance wl needs is lightning. I usually take 2-3 seconds to kill a t5 herald soul, so nothing that a few rejuvs cant solve.