What the XP supporters don't realise is how much extra work it is to support XP if you don't do it from the start, you need to write an entire new render path, new shaders for DX 9, plus all the testing and troubleshooting, all to support a minority of customers on an OS that isn't even fully supported by Microsoft any more.
Windows 8 is out next year, seriously it's time to drop XP.
It's true that only 20% of people on the Steam surveys are on XP so cutting them off isn't a disastrous decision but where it has wound up pretty much is. Fewer than 10% of users have a card capable of running Battlefield 3. Supporting DX9 from the off is still the only decision that makes sense because there are still just so few cards capable of pulling significant DX10/11 effects at anything resembling playable speeds.