If GL_NV_depth_clamp will be added to the core, wouldn’t it be useful to enabled/disable the clamp mode for the near and far plane individually?
I currently use depth_clamp for shadow mapping to avoid near plane clipping of shadow casters, and keep the depth range focused on where it is needed most. However, I don’t want stuff clamped to the far plane. If distant geometry were clipped by the far plane and not rasterized at all (as normally) it could save a tiny bit of performance. And I’m sure there are other uses as well.