I just started getting into gaming a few years ago and still have much to learn. Everyone has to start somewhere. So, would you care to "put your money where your mouth is" and enlighten me?
http://en.wikipedia.org/wiki/DirectX
In layman's terms, each version of DX brings more functionality and options to what can and can't be programmed, particularly graphics wise such as shaders, lighting, particle effects, etc ( although DX11 can also bring great performance improvements as well as graphical ).
But DX by itself does nothing without the hardware to support it.
For example, I've lost count of how many computer "noobs" I've seen asking on boards why their PC couldn't play Halo 2 because "Halo 2 only requires DX10 and my Windows 7 comes with DX11, so it should play fine!", yet their cards are only DX9 cards. They think because Vista/Windows 7 has DX11 support, that means they can automatically play DX11 games ( of course, most of these people don't even know what a graphics card is and think their DVD drive is the cupholder ).
You can add DX10 and DX11 options to any DX9 game, all it does is give those with DX10 more graphical options than DX9 and those with DX11 more graphical options than DX9 or DX10. But it does NOT remove the ability to play the game in DX9.
But I would highly suggest anyone who wishes to continue gaming into the near future buy themselves at least a DX11 card, as DX12 will most likely be released next year ( as Windows 8 is being released this year ).
Devs have stood by DX9 because the majority of gamers still use DX9 cards, due mostly to the fact DX10 was pretty much a flop due to both developer laziness and Microsoft's own corporate stupidity ( feel bad for the poor guys who bought the very first DX10 cards, only to find them completely useless when MS made DX10.1 the DX10 standard ). But even now, DX11 cards are more than affordable ( the GTX 460 I paid almost $300 for can now be found around $100 for example ) and becoming commonplace.
And it's going to be hard for dev companies to justify sticking to old DX9 when DX11 cards will be the common card and DX12 cards the new "performance" breed. Chances are, DX12 will support DX11 and maybe even DX10, but not DX9 as the architecture of the code will just be too different.
A couple examples of games that have had DX11 added after the fact are Crysis 2, Lord of the Rings Online, and World of Warcraft. You can still play them with DX9 or DX10 cards because they were originally programmed for those API's, thus the code is already there to support them. You just can't use the added features DX11 brought to those games without having a DX11 card.