don't forget the part that anyone who goes on your land and is smart alek enough to punch one of the iron beats will most likely be exterminated by the golems...
also, don't forget the part about server flooding, iron golems probably cause more lag compared to the simple villager or pigman, if someone (*looks at ronvandorp*) creates a machine that would spawn hundreds of them at once could corrupt chunks or even crash the server.
Then again, it could be a another reason that relates back to the economy, its something we have to fix, and a new spawner in the market that can cause people to create ores of certain types should not be sold, especially when there are people like ron in this world which will create an extremely efficiant design for a invention that lags chunks out completely.