You will get many ideas regarding the "need" for antivirus and malware protection. Many people here are seasoned users who recommend against it, largely based on what they consider easy steps to either avoid or detect malware presence; however, (and in no offence to them) these steps are not always obvious to everyone, so it's largely a subjective matter.
I recommend you give Thomas Reed's Malware Guide a read: http://www.thesafemac.com/mmg/
This will give you a pretty good rundown on the details of the Mac malware scene and considerations for security software.
Overall, however, a lightewight and free anti-malware utility will not hurt your system and can be set to only scan periodically to help you detect any known malware with minimal impact (if any) on your system. Unfortunately malware does happen, and while it is still rare in OS X and while does have a number of built-in options to help prevent it, the possibility is there and security software is simply another tool to help manage it, should you encounter it.
Some such tools are Sophos Home Edition, and ClamXav. Others will work as well, but you may have to limit some of their default behaviors as they may try to activate full-heuristic scanning routines that can sometimes be a touch intrusive.
Don't use your Mac stupidly on the internet.
Don't install things from shady websites.
Don't just enter your admin password when some website says you need a codec or plug-in to view their content.
Don't use p2p file sharing.
Be able to recognize phishing scams. They're easy to spot because legitimate organizations will not ask you for account information in an email. They will tell you to access your account via your normal web login, not provide you a link, which will be a fake website to mine your personal info.
Market share has little to do with it. It's unix that makes it secure, but you can still allow yourself to get fooled through social engineering attacks.
The built-in protections will prevent any existing known malware from being installed.
There is no know Anti-Malware software that can detect a zero-day attack, which is one that nobody currently knows about except the attacker. Your brain is the only defense against that. Don't be stupid.
Thank you for your very informative post. What is your position on the need for anti-virus software for Mac when running a virtual Windows machine via Parallels? Do you consider running anti-virus software (specifically Avast!) for Windows in the virtual machine only to be a sufficient safeguard?