I think the answer to the question 'does Linux need a firewall'? is simply, it depends. Linux is inherently more secure than Windows 'out of the box' and comes with iptables as part of the default configuration. Firestarter/Netfilter are simply front ends to iptables, essentially making it easier to set-up your rules.
Do we need one? If you want to make sure you have covered all the security bases, then yes. Also, if you use p2p or wish to have some control over ICMP then also yes. Finally, if you're running a server, then you most assuredly will want one.
The same can also be said of AV and AS. The short answer is no we don't need them. Don't forget, because of the way permissions work under Linux, virtually everything we do under Linux is as a limited user. Executing a file typically needs either root or sudo permissions, perhaps with the exception of the home folder. So for the most part, unless a malicious script aka a virus or some malware can gain those necessary privileges, they won't get very far.
As more and more games become playable on Linux, people who are fed up of paying hundreds of pounds/dollars/yen, what ever, primarily for an OS so they can play those games, will see a powerful, user friendly, FREE OS, like Linux as extremely attractive. After all, it can do everything Windows can do now, except play games as well.
