He's talking about trap sight radius, not trap affinity. You can get through the game pretty easily with trap affinity of 0 (not being able to disarm traps isn't that bad, since you can just walk around them), but if you can't see them until you're one square away from them, you will step on them. A lot. And they will kill you, a lot. It only takes one thaumite nest at the wrong time to seriously mess up your chances of survival, especially if you're not a tank.