Leprosy is not just micro management. Even a single cast against a large army can death sentence 1000g worth of mages and many gems worth of summoned elites. Cast that 3x a turn, every turn on an invader and if you can stall him for a few months, he's completely screwed - unless he is just totally outplaying you otherwise.
Even at 10 gems a player with a small stockpile can ruin an enormously expensive army at relatively little cost - at 5 gems that ruination is easy just off of your monthly income. As if Death nations needed less expensive tools of destruction.
The Demons are a complicated issue - I do cast the individuals sometimes in Vanilla, but only in rare cases, otherwise I shoot for the batch summons if I intend to use them in my strategy. But 2-3 slaves apiece is just wow for some nations.
For example, with a strong Blood income, over 14 turns you can make a raw Empowerment + Booster (total = 68 slaves w/hammer) outperform base batch spell casting. So if you get Blood started early, and take a long view, even a nation without cheap B+? mages can do better with the single casts at 2 slaves per demon. Even 3/demon isn't too bad, it's not until they cost at least 4 apiece that it's just not worth it in a reasonable amount of time.