I downloaded the Vanilla server jar to a fresh directory. Started the server to generate the files; Set the eula.txt to true and the server.properties difficulty to normal (restarted as needed).

I then connected, opped myself and set my gamemode to creative. I did /time set night and waited, observing what I would consider a normal amount of monsters (zombies, skeletons, and creepers) to spawn.

I then stopped the server and deleted all the world files, library folders, etc. I retained the server.properties file with difficulty=normal.

I placed the Spigot rc3 jar in the same folder and started Spigot. Again, I connected, opped myself and set my gamemode to creative. I did /time set night and waited through the entire night and didn't see any zombies, skeletons, or creepers.