While I may turn normal maps on in the future, if the normal map effect was scaled realistically you probably wouldn't be able to see any difference because while the moon is 3475 km across the highest mountain on the moon is only about 5km tall. This is a ratio of 1/695 so if the moon was 700 pixel across the mountain would only be a single pixel difference in height.http://en.wikipedia.org/wiki/List_of_mountains_on_the_Moon
It would still be cool to turn it on and make the map effect very dramatic.
in reality the height scale of features on the moon wouldn't be visible to an observer
It is true, of course, that the moon would still appear to be perfectly spherical but
the height differences would make themselves known through light and shadow - as they do on actual photographs of the moon.
Enabling normal maps would allow one to make a diffuse map without any intrinsic light and shadow effects, and then have those provided by the normal map, so that they'd change dynamically with rotation of the object in US2. Also, on smaller objects such as asteroids, the height differences would be more readily visible.
I'd certainly love to have the option
EDIT: My point is not so much to have the moon model appear more realistic, nice though that might be, but rather creating new textures with normal maps.