I created a Terrain Importer for Unity3d recently that has been released on the Unity Asset Store for $5. It was primarily built with World Machine in mind, and can import tile sets (even large sets that exceed the 3.5 GB limit of Unity’s Editor can be imported).
You can certainly use the default import method to import your height map and then use the free splat map import tool to import your splat map; you’ll just have to do so for each individual tile, which may not be a problem if you’re only importing your tile set once. If you find yourself making adjustments in World Machine and needing to re import the height/splat maps into Unity, it can get tiresome.
So no, my tool is not required, but it makes the process faster and easier.