One thing I should mention is that the relighting technique in Nuke only changes the diffuse lighting.
It can't change shadows or global illumination.
So, you can change the diffuse lighting in Nuke. But by replacing the diffuse element with the relighting setup, you wil lose any shadowing.
And global illumination is based on bounced light from the original Terragen light source.
So this technique is better to make tweaks to the lighting rather than radical changes.
Another use might be to animate subtle patches of light over a terrain to simulate the effect of cloud cover occluding sunlight overhead. That way you could take a still image rendered in Terrain and create an animation that has some "life" in it.