Yeah, but what if you need to take back his mines? After all, practically all resources, except for wood and ore, are guarded. And you can't take back mines with a mercenary hero's army. And by the time you get there with your main army... U-u-u-u-u... The computer-controlled enemy can (and most likely will) grab the allied city and its mines much sooner than you. And that won't be good.

Well, let me show you the difference:

1. If your ally takes the mine, it will remain inaccessible to you, and all the resources will go to his treasury. How much of it he will send to you is unknown, but it won't be very much, I think...

2. If the enemy takes the mine, you just need to visit it, and it will become yours - and the ally won't be able to repaint it in his color anymore...

So, the faster your ally, if you're lucky enough to have one, disappears, the better the result will be for you.

And one more thing. I, for example, have noticed that allies often create small obstacles - many times they blocked passages with their heroes and hindered development. The enemy does the same, but that's understandable - he's the enemy, and you can punish him for it :) But with an ally, you can't do anything - you just have to put up with it... :)