12 Population for a 2x2 production building? It's the most effective by far for the repetitive productions - and it's not even a close contest. It leaves both space (which is usually at an absolute premium) and population free for military and/or Goods buildings.
As I said, blacksmiths are not the most effective 'on all counts' because you're ignoring the actual reason supply buildings are continually introduced to the game - because they produce higher and higher values of supplies. By that metric, blacksmiths are very ineffective.
For what it's worth, I'll provide my own perspective to your implication that goods and military buildings are more valuable than supply buildings. Goods can be generated from recurring quests in pretty respectable quantities, and goods buildings are NEVER needed for quests (I've yet to see a quest ask for goods to be gained from an actual goods building, only to obtain goods using whatever means you wish). Side note, I've also completed all OF research and have thousands of each OF good in the bank without ever constructing an OF goods building...and my Chateau is only level 11. Imagine how high those numbers would be if I'd actually gotten around to 'super level' the thing. Basically, it just doesn't seem like a good use of space for me to have goods buildings because they don't actually accomplish anything.
On to military! With Alcatraz, there's likewise rarely ever a need for me to have barracks for a prolonged period of time. I put down each OF barracks for a little bit until my level 14 Alcatraz got me up to 200ish unattached of each unit, then sold them back off. It's been months for some of them, and only Turturrets are down below 170. Fighting with attached troops is kind of a relic from a time before Alcatraz was added to the game and the ability to level GBs beyond 10 became a strong reality thanks to the Arc. Even GE can eliminate the need for barracks by providing up to 24 guaranteed unattached troops every week. Granted, you can't control what you get from GE but at least in OF, I use every unit at various points so nothing ends up being useless. Essentially, beyond a brief initial period where I use Alcatraz to give myself a stockpile, barracks are never a mainstay in my city beyond the ubiquitous rogue hideout - which side note, can always be used to place 'empty' armies in GvG to slow down enemies in a pinch so there isn't even a need for a monstrous age-appropriate barracks in all cases either.
Anyway, while it's probably not going to convince anyone who's got their mind made up, there's my own perspective on the value of supply buildings in one's city. Goods buildings are irrelevant, military buildings are only temporarily useful, and supply buildings always have a use for how I play the game. Blacksmiths aren't going to cut it, so I have space to spare for stuff that's age-appropriate.
