Currently, people are able to create monster iron farms in spawn chunks which produce thousands of ingots per hour.
This is due to villages not merging together because they are in spawn chunks. This behavior seems unintended since anywhere else they would merge.
An easy fix would be to reload spawn chunks like once an hour so crazy stacks of villages merge and break these exploit farms.
Is this still an issue in the current Minecraft Snapshot 15w46a or later? If so, please update the affected versions in order to best aid Mojang ensuring bugs are still valid in the latest releases/pre-releases.