ccgnick asked: Why not change what represents "invisible" in flavor to, "This creature can't be blocked except by walls/creatures with defender"? It would make walls much more appealing, for one thing.
Walls don’t just block every creature. (Note that I’m not even a fan of walls as creatures because they don’t make much flavor sense as living beings.) Walls block because their controller (you) move them to block. If you don’t know to move them, they aren’t going to block the creature in question.