Hi Domdib,
Here's a small comparison between using a regular surface layer and a displacement intersected surfacelayer.
[attachimg=#]
IMO the version with displacement intersection enabled looks much more realistic. On some parts the grass-layer creeps over rocks where the flat layer does not.
It also gives a stronger idea that the slope restriction is more randomized, (edit) because on some slopes the flat layer has coverage and the intersected does not. (/edit)
Cheers,
Martin