Monsters NOT DESPAWNING even when out of chunk range

This is a bug that has been in every version since about 1.6.
The problem is that monsters don't despawn when out of rendering range. For example: I can go to an area at night and have monsters spawn around me, then I leave, while it's still dark. And by leave I mean go out of chunk range. Any number of Minecraft days later, I can visit that same area again, middle of the day, and all the monsters that spawned [x] days ago will still be there, and won't start burning up/despawning until I can see them.
The problem with this is that the spawning/despawning code is not working properly, and I think it might be causing unnecessary lag.