Minecraft wiki entry for Piston_uses, associated text: To blend in well into the environment, you need a stone or dirt wall. Stone walls can be found on the mountains biome (and its variants).
If you need to build it on a plains biome or other flat terrain, you need to somehow blend the door on the environment. If this is impossible for some reason, the only choice is the hidden piston stairs.
Players can accidentally mine the door, as no one knows there's a door on the wall.