Like the OP said, there are certain situation where the light is a must and there still has to be a shadow cast onto the underside of the terrain.
Also, a box is 1 drawcall. The 2 meshes are 2 drawcalls and have a high margin of error.
The height of the mesh would need to be as long as the sun declination ratio to the landscape length.
With a box, you use one drawcall, and it can be 1m tall without having to do anything at all.
It’ll just cast a shadow into the underside, no matter what