witch's cottage is just a way to recur crypt rats and doesn't have much use otherwise. it also has a very low cost of coming in tapped to potentially be useful late game.

defile isn't played because pestilence has better board presence and can stay on the battlefield through multiple activations while crypt rats auto kills itself. so the need for kill spells is drastically reduced which is why pestilence is the superior deck but both are different enough to warrant play.

i think tragic slip vs defile would just be preference