Second reason is too keep population growth high, a happy population grows more quickly, and during times of war when you need to recruit troops quickly it’s very important to be able to get a high population growth rate. Another thing when it comes to wine and war is that it’s much much harder to beat down a player with lots of wine cities, it’s almost impossible. Even if he loses all his troops he’ll be able to recover real quick since he’ll still have max population in his cities, he’ll be producing and consuming wine even if you plunder his other resources after all. A non wine city will have a much harder time keeping it’s population if it’s being plundered often since it depends on getting its wine sent from other cities.

If you’re in a peaceful neighborhood, being a peaceful player that and just sticking to yourself I guess having a mix of all kind of resources is ok, but for an active plundering player that risks being attacked back during night, wine is definitely the superior resource. If you worry about raids during night just set wine production to the same as you consume, stop producing all other resources, and you’ll wake up in the morning with lots of  gold and no losses. I’ll write more on this in my wine and plunder article.