Quote: "Um, what the difference in the end?"
The difference for terrains is that instead of sending thousands or millions of vertices to the graphics card for transformation and throwing all but a few away, you send only what you know will be rendered, saving all of that valuable GPU memory and processing power for other things such as the objects where it makes sense to use polys.
The later Novalogic games work in this way - in Landwarrior, terrain is generated using voxels, while buildings, objects and men are poly-models.
Quote: "Anyway, with DX9 ... and the vertex shaders will increase the polygon count"
I was under the impression that shaders for DX9 can't create polys, but with clever pixel shader tricks, can give the appearance of extra detail.
Now if you actually meant DX10 shaders, which I don't have access to, then you might be right. If that is the case, then something to think about is that it's only taken 7 years from the release of LandWarrior where you have the same potential for generating detailed terrain
Now I don't believe that voxels are the greatest thing ever, or can replace everything that can be done using polys, but I do wonder where we'd be today if more attention had been paid to voxel tech. Unfortunately, all the attention is on getting the next game out of the door as quickly as possible, so I guess we have to live with that.