Well, that's practically looting... :D
Also, it turns out that the ally can't develop and, therefore, will be quickly defeated...

The sooner this happens, the better for the player (if the ally is an AI, of course). Because then you can capture the former cities of the ally and use them for yourself. And you can't do this directly according to the restrictions built into the rules. So, weakening the ally is in the player's interest if they are playing to win...

The benefit of an ally is small - they reveal the map and visit obelisks, which, if I'm not mistaken, also become "visited" for you (but you can do this yourself if you recruit heroes in the ally's