Actually, market share based viruses and malware is a completly valid argument.
Sure, there is the occasional virus that is just there to cause widespread havoc, but alot of viruses and malware are aimed to gaining control of the target computer to convert it into a zombie and create botnets. That is why spam is so hard to get rid of, there is no centralized server spamming the entire internet, it is a botnet that can be commanded to spam, perform ddos attacks or just sit by until activated without the end user knowing anything about it.
From a business standpoint, it is way more cost effective to develop a virus that will target 80% of the consumer computers.
Yes, *nix systems are usually less prone to viruses and attacks, but that is usually because *nix systems are used as server platforms, where users only use a few services such as ftp, ssh, virtual machines, web servers, etc. Each of these services could be potentially exploited, but on the server side, there is no user downloading from limewire, torrents, or shady web pages.
Put a power luser behind a *nix system, and he will grant root access to just about anything because he doesn't know any better and voila! You have a new backdoor, trojan, virus or spyware installed.