Many players are farming resources in custom maps because it's easier than in a story map.
But since 1.8 they always have to check whether the miners are mining actual resources or just for score because they cant see easily if a miner needs to be replaced to can get more resources or not.
Pre 1.8 miners stopped mining and had a gray colour instead of the colour of the resource it is mining. But now the player always has to click on the miner wich is pretty annoying when having 15 active miners.

So the suggestion is that there should be an indicator on each miner wich shows if it's mining resources or not.

But please before working on this, fix the inventory problem mentioned by many other players on the tracker.