You mean using DirectX 11, but only using DirectX 9 effects? That's easier to do, but still requires two different versions of the game (two different .exe's). Some of the techniques used with the DX11 renderer may optimize the rendering of DX9 techniques, but those optimization techniques still don't work on DX9 cards. It'd be easier for them to do and I'd like my games to run a wee bit faster. But this helps a very small minority of people. You'd really need the worst kind of DX11 card if you have trouble enough running a DX9 game that you need a DX11 version just to make those DX9 features faster. If you're gonna make a DX11 version, you may as well go all the way with features or else you're not helping very many people.
I've seen plenty of games have both DX9 and DX10/11 modes in the same executable, just requires a restart so that everything is set up properly. Anyway, it's a free performance boost, and in the case of skyrim, will give modders more freedom, so why knock it?
Of course, it'd be nice to have all the shiny effects in from the start, and have skyrim *actually* look like a "next gen" title, as we were told it would, but I'll take a visually identical DX11 mode over nothing at all.
@CCNA; That seems an incredibly cynical way of thinking about it. TES games are praised for how immersive they are, and that factor especially can be improved with better looking graphics. Tessellation especially would be both a huge performance boost and a huge graphics boost, if done right.