Does normal mapping make sense for a view of earth from space?
In my opinion, yes, it does. I've heard it claimed that in reality, the ratio of the tallest mountain on Earth to the Earth's diameter is smaller than the variations in height of an apple's skin to its diameter. So I can't say that it would be realistic, but it certainly looks better than not doing it. At the least it gives you some shading between high and low spots, even with a simple directional light.