Zerg egg mechanic.

Hives produce larva at the same rate as Hatcheries.

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Remember that time itself is a resource. Spawn Larva is useful because it gives the Zerg free time instead of free minerals. With time you can buy minerals, or pressure on the enemy.
Also, Drones will last, while the Terran/Protoss mechanics wear off quickly.

 

Yea but that forces the Zerg to max out most of their larvae with drones and throwing the drones at every expantion spot they can take. It would also mean that every expansion that is taken is felt as a huge kick in the nades. For every fallen expansion you loss masses of drones that you need to keep mining pace with the other races. And just thinking the other races especialy with the economy boost can easily keep pace with the zerg being low cost and have just as many expanssions as he Zerg player with the economy boost at each one.

This forces the Zerg player to have to expands as often as posible if not even more then is posible to keep up with the economy boosts of the other races.
(but it's all just a matter of time. Blizz seems to be working on the other races a little bit more then the Zerg. Other then the masive new renders that they have done with the Zerg they seemed to have only done that and no more.

But maybe there could be an upgrade at the hatchery that allows the queens spawn larvae to be at a smaller amount of energy. So instead of making it only 4 times with max energy it's now 5 times. And combined with what I said another thread (about the queen growing and getting more energy per mutation "it was in the curren last page of the queen poll / guild thing.) it would revolutionize how much units the Zerg can make in a short amount of time.

 

+1 on upgrading hatchery in order for it to produce 4/5 larvae instead of 3 or have it spawn larvae faster XD