The corpses are disappearing in double spawn modification cause it's drops performance a lot. Each corpse is a skeletal physical body and each part of corpse like head or leg is skeletal physical body too. When monster dies he spawn particle effects and blood decals. When you kill a hundred of monsters they spawns a hundreds of objects. And even if those objects is staying behind the camera and not rendering, and physics is sleeping. Those objects harms performance a lot because they stay on map. The existence of these objects in itself carries a high load on the processor. And by the end of the map, when a lot of corpses accumulate, the fps can drop from 100 frames to 50 or even less.
This is why this options is disabled.