...but that's just retarded.
I am a Linux man myself, but no system is invulnerable. It is insane not to use the firewall system provided to secure your PC. There are security vulnerabilities discovered regularly for all pieces of software, including FOSS.
I admit that their are few Linux virii in the wild, but they do exist. Also, you could potentially forward on an email containing an infected attachment to one of your mates unknowingly. When a free AV (such as Clam) could scan your email, and use very few resources doing so, I don't see why you would not do it. In adition, Linux virii will likely become more commonplace as it gains more of a following, so as time goes on your chances of infection will increase (and they are not zero right now).
Anti-spyware, I'll grant you, is not as big a deal. But the others... I must point out the huge FAIL in your decision.
Don't get me wrong, I agree with the argument that Linux is "more secure"* than Windows, but only providing you use the security facilities available.
* "more secure" in quotes because it isn't the right phrase to use, but ICBA, it'll do, take it with a pinch of salt