I think the spawn limits should be removed. Their is no need for it. If you want to place down creatures until your game lags. Why not? It's your game.
It's not about the creature you might want to place or breed yourself... it's about the ones the game spontaneously spawns. If there were no limit at all, then the game would just eventually lag itself into a total freeze up.
To be fair, the limit is really high. It's at least 400 for passive mobs, from what I've read. And, I've had enough Endermen to get past level 30. As stated above, if they removed the limits then mobs would pile up and lag your game unless you went out and killed them or caused them to despawn regularly.
Why not?