Well, well dx9 would choke up if it would try to make same effects as dx11 version, but it does not chance the thing that differenses are hard to notice. Is dx9 good enough? Well it's not near enough to reality. DX11 has more muscle to do that, but we need even more power than that. The ability to create more details via tessalation with dx11 is a good thing. 5870 is fast enough to run the game in dx9 or dx11 mode, why not use dx11 even the difference is not so important in fast pasing rasing game.
It will take time (again) to developers to really crasp all the benefits of dx11. Adding a feature just for adding the fealture is not a sure way of making the game better, they have to learn what improvements does count the effort!