There’s a million reasons, really: enhancements to your computing experience and bullshit you don’t have to put up with. One of the biggest checks in the latter column is Conflicker.
From theatlantic.com, an awesome article on the most prolific and tenacious Windows worm ever created. Despite the efforts of some of the world’s smartest coders, botnet experts and cryptographers:
As of this writing, 17 months after it appeared and about a year after the April 1 (2009) update, Conficker has created a stable botnet. It consists of anywhere from hundreds of thousands of computers to 12 million. No one knows for sure anymore, because with peer-to-peer communications, the worm no longer needs to check in with an outside command center, which is how the good guys kept count. Joffe estimates that with the four distinct strains (yet another one appeared on April 8, 2009), 6.5 million computers are probably infected.
The investigators see no immediate chance or even any effective way to kill it.
Basically, no one knows how many computers are infected, they have no idea how to kill or even quarantine it and have no clue what the worm’s creator(s) ultimate intent is.
Obviously, Macs are unaffected. Sleep tight, Wintards.