I have noticed in my creative world that mobs don't despawn even if I go really really far away. I believe these mobs that stay spawn makes the spawn cap stay large and create issues on just about anything that relies on mobs spawning normally.

What I expect is that the mobs would despawn if you are far enough away from them and would increase the challenge in dungeons and caves because the spawns would be in that general area.

Right now I can walk around at night and only need to kill a handful of creatures while working on my base which isn't spawn proof... I don't even think that putting the game on peaceful right now despawns creatures when they are far enough away.