Generally speaking, DX11 offers more eye-candy. It is also more demanding on your video card.
Therefore, if you were to compare playing a game in DX9 mode vs. DX11 mode, you will notice that overall performance is lower in DX11 mode, but the graphics will look better.
Support for DX9 by game developers has recently started to decline. I forget which game, but there was a recent game that required DX11 (not playable with Win XP). Much like how Oblivion marked the demise of DX8 since that was the 1st game released which dropped support for DX8.
Based off the newest Steam hardware survey which actually covers the vast majority of gamers, 62% of systems check had a DX10 GPU and were running Vista/7. The results tom's quoted were for window's systems overall which also includes office machines that no one games on that were never upgraded to vista and wouldnt have a real GPU anyway, the Steam survey is a bit more relevant. http://store.steampowered.com/hwsurvey
You dont get people to switch over to the newer better stuff if you keep programing in support for the old stuff forever, it also makes it a much heavier program, if you only have to work on DX10 you can work on optimizing it more than if you have to optimize both DX9 and DX10 so its always good for some things to start dropping support for the old tech for the sake of everyone.