Besides, in large maps you will have a lot of cities on idle a lot of the time. If you miss one of the cities that you DO want to build in going into idle mode, you will probably forget about it for quite a lot of turns which might cripple your game easily.
It hasn't been just once or twice when I have forgotten a city for 20+ turns just because I missed the notification that the city has become idle. It just doesn't make any sense NOT having a popup that asks you once whether you want to keep the city in idle or build something. It's not like you're not going check out the city anyway after a building is completed to see if you want to build more or not. The popup wouldn't slow the game at all, in fact it would speed it up at least in my case.
The lack of a popup leads to tedious micromanagement of the cities. Give us the popup!