I think the OP's question is kind of a moot point as OS X already has antimalware built in, i.e. XProtect. I agree that using safe computing practices is the best thing one can do to protect oneself. But let's be realistic in that every instance, we cannot expect everyone to follow best practices when it comes to using a computer. Installing AV software is one way of compensating for this shortcoming.
I think it's also worth pointing out that Apple could do a better job of making OS X hardened better against threats out of the box. For example, they're first use wizard could walk users through creating non-administrative accounts for daily use, and the built-in GUI firewall could be enabled by default (and the GUI firewall could include more sophisticated configuration options as well).
With that said, I think GGJstudios's contention that by always following safe computing practices will result in a 100% malware-free system, or to fixate on one particular type of malware (virus) to the degree you are doing without talking seriously about others, while ignoring the very real known vulnerabilities that exist in OS X, is irresponsible. Despite all the security-related press that Apple have received in recent years, it is unconscionable that Apple still includes vulnerable versions of software in currently-supported versions of OS X (Apache comes to mind). Things like Apache, bash, ntpd, etc. are all components that are ripe for exploitation under the right circumstances and the more technically-inclined folks around here need to keep that in mind.
This is not true. The ntpd vulnerability is a recent example of how one can execute arbitrary code remotely on a Mac with root privileges. Note: it is not necessary to run ntpd as root, but Apple does it anyway. This is bad practice!