Here's some visual comparisons of DX 10 to DX 11 for you to look at:
http://tech-mania.com/wp-content/uploads/2009/09/call-ofpripyat1.jpg
http://1.bp.blogspot.com/_3oLB6Otdlcs/TKc1CNt6xuI/AAAAAAAAIc4/XLGQRRsii6Q/s1600/dx10-vs-dx11.jpg
http://i45.tinypic.com/14mh0cl.jpg
http://i50.tinypic.com/2n691c8.jpg
http://i45.tinypic.com/21jpyfm.jpg
http://i50.tinypic.com/2luwe90.jpg
Imo, tessellation looks wicked. Although it can also be too much on a few rocks...
Still, awesome
Skyrim development begins 2007 + DX11 releases 2009 + Beth dev cycle half over at release of DX11 = No DX11 for Skyrim.
TES offers the gaming industry something that no other game on the market can compete with and since it will blow Oblivion away it doesn't need DX11. It just doesn't.
New DX support can be added at any time, no need for it to be added at the beginning, in case you didn't know. That's what Timeslip did for Morrowind.
DX 11 isn't needed, but there really is no reason not to include it. It's not like it's the biggest task in the world.
DX 11 helps a whole lot with performance and adds many many new graphical possibities. Why wouldn't you want DX11, tell me please?
The technology can be added without too much fuss, and the technology is a lot better than before. Not to use it would kinda be like cutting yourself in the ancle, causing you to slowly slowly bleed to death.