it's very common in other game makers, they call it as tileset, where you'll have the corners, middle, bottom and top with different contents, when you paint the scenario with the selected tile, it'll detect the corners, the sides, the middle and will fill them with the right image.
I think C2 is good with his simple system, because doing the tiled system of RPG Maker don't give you the freedom to create the scenario in Coreldraw and import it, for example.
I prefer to manipulate mosaics outside the Game Maker software because revisions show be ease than edit them inside. But it's a personal preference.
But, C2 really need improvements around the layers and Z ordering, or Our job will become hard to maintain, for example is when you edit a middle ground on a finished scenario, you can't select it if you place grass over it, so, in complex scenarios, select a smaller thing between 3 or 4 objects can hurt your mind.