Outline shader being shown only in the material editor

I’m trying to make a simple outline shader I got from the wiki.
The shader seems to work, but is only being shown in the material miniature, but not in the editor nor the player.

If that shader creates an outline by extruding backfaces of the mesh along their normals, it won’t work for a simple cube that is axis-aligned to the camera. Try applying it to a more complex 3d mesh.