Image By WOTC For cubes, you want most of your cards around the two- or three-mana range, with the number at each mana cost declining as you go higher
Then, because it's hard to make a typal deck work when you don't have as many creatures to choose from, we gave the Reaper King a very powerful ability
However, its in human nature, that you dont want to risk the worst thing happening, so you tend not to play cards like this
You can not only prevent lifegain for your opponents, but you could also put a land onto the battlefield, give you creatures +1/+1 and reach, or deal three damage to each opponent at the same time
Regardless, this is a wildly powerful interaction in Standard, particularly if the Books static is able to be taken advantage of