Looks like you guys dont really get the point, so I'll try to do my best to resume :
Vue, Terragen, WM, GC ... none of them can compute 3d erosion, they all do erosion on 2d elevation data. Type erosion in google images, you'll see tons of example of shapes that 'simple' elevation cant produce.
Voxels are great for terrain sculpting right (Crytek ?). Vue has a terrain sculpting room (very limited).
3Dcoat would be a really good solution too, especially with some procedural filters for large scale terrain creation. A 3D erosion filter would be one of them.
I have not a single interrest into hair, open suvdiv, nurbs auto retopo