At the present time it's a waste of time, resources, and money. Now, what the future holds is anyone's guess. It could be that tomorrow things change, and an AV program becomes as mandatory on a Mac as any PC running Windows, but for today, it's not necessary.
That's not to say that simply using a Mac means you can act with impunity on the Internet. There are still plenty of threats that are very much platform agnostic, because they rely on user stupidity.
So, you don't need a dedicated AV program, and most of the ones for the Mac exist to cash in on poorly written policies at large companies where all computers MUST have an AV program... Regardless of whether or not one is actually needed by the platform. That's what you get when HR types try and write IT policy. Anyway, just don't take that to mean that you're immune to any and all threats out there simply because you're using a Mac. You're not, and you'll find that out the hard way if you start thinking like that.
Also, be mindful of the fact that this situation could change at any time. So, be sure to survey the situation every now and then, and be ready to act if and when things do change.
I know this is probably an age-old question but I can't seem to find a definitive answer on the internet. Are there any real threats out there for my Mac OS X or is installing an anti-virus just a wiste of time, money and memory? Should I be concerned about passing on viruses through e-mail or USB stick to my colleagues or friends who use Windows? If so, which anti-virus should I use? Is there any real competition between anti-viruses out there?