While I imagine that around settlements the corpses of the recently deceased would be cleaned up, and some in the wilds would be consumed by wildlife, it would add a nice sense of realism, in a macabre sort of way, to fight a battle in the middle of nowhere and wander past the site later to find the dead bodies turned into rotting corpses and later on skeletons. Maybe give corpses a "life-cycle" of a few weeks, or a couple of months, where they transition through the stages.
The models are certainly there for different stages of decay. I would even suggest that they may have been included specifically for modders to add, since it is something that would probably lead to a higher ESRB rating... but what modders do has no impact on that.
