Bruce Schneier explains why Windows XP SP2 should be available to unlicensed Users:
The security of your computer and network depends on two things: what you do to secure your computer and network, and what everyone else does to secure their computers and networks. It's not enough for you to maintain a secure network. If other people don't maintain their security, we're all more vulnerable to attack. When many unsecure computers are connected to the Internet, worms spread faster and more extensively, distributed denial-of-service attacks are easier to launch, and spammers have more platforms from which to send e-mail. The more unsecure the average computer on the Internet is, the more unsecure your computer is.
I couldn't have put it better myself and the rest of the article is worth a read too. However, in my opinion, he goes off the rails when he states that:
SP2 is an important security upgrade to Windows XP, and I hope it is widely installed among licensed XP users. I also hope it is quickly pirated, so unlicensed XP users also can install it. For me to remain secure on the Internet, I need everyone to become more secure. And the more people who install SP2, the more we all benefit.
Via Dana Epp