If you waited 18 months after the debut of each version of Direct X to buy a VGA card, you'd either never buy a new card, or always buy a VGA card about 6-12 months before the next version of Direct X came out.
While it's not particularly intelligent to buy a VGA card specifically for DX11 compatibility when there are only a handful of DX11 games currently available, particularly the enthusiast models that command a premium, it's not particularly intelligent to wait almost until the same card is on the verge of being cut from production either despite the big difference in price unless price was the primary feature being shopped for.
Memories run short, but the EXACT SAME argument was levied against DX10 back when DX9 was the standard.
Unless you take this to mean skip every other version of Direct X when it comes to upgrading video cards DX11 compatibility is a good feature to have in a current VGA card.
The current GPU generation is a bit unusual thanks to Nvidia's Fermi delays, which meant prices on premium DX11 cards from ATI did not have to drop as they would have had their been any competition. Average price for the 5870/5850 is still $400/$300 or same (often more) as when they debuted 5-6 months ago. Still in "short supply" too.
A good time to buy a premium card is about after manufacturers drop about $100 off the original prices, usually less than a year after debut.