I never use manual, although I will shift P for pollution control. Late in the game, when you have a million workers, and would like to use manual, you will find that the jerks go around replacing irrigation with mines and starving your cities, and if you dont catch them in the act you will never find them and they will do it again and again. Don't you always have a road to build in a certain direction? Or a city that needs to be surrounded by mines so it will pump out a wonder? Or mines to replace with irrigation so you can keep growing? Or a jungle to clear to stop disease? The AI has no clue.