I am guessing due to the fact that drakes in essence cannot be prevented from entering a defensive perimeter, because they constantly roam without being aggravated, preventing drake spawn is not a game mechanic. Drake will also attack a player's base in their absence. I recently used a mountain biome portal I had not used in over a month, and after exiting my cabin I found all my banners had been destroyed. There was still some materials from one laying in the snow.