Yes, please do that! I think an empty terrain is not really something you can use for serious stress testing. There should be a decent amount of grass, rocks, vegetation and multi-texturing to see how it works in a real game environment.
And then there is still the option like you saw on udk demo to animate foliage, to show particles, moving clouds, animated water ect.

And it is not always needed to have the best and high poly vegetation. The lighting is way more important to achieve something more interesting. Just look at the UDK demo, the terrain and vegetation (though quite low poly) looks, like it really gets the light colour from the sky. But you will see from the A7-images of DarkInferno, that the terrain textures are way too saturated and do not reflect any colour and light from the sky. It looks not real, rather cartoony.
And shadows of course can add a lot to a good scene, like you saw from the Leadwerks video.

I already made populated terrains in Gamestudio and you will see, it is not easy to place tons of vegetation manually. After some weeks of hard work I just gave it up. But maybe some new user-made tools can help here.