Yes, some do. Many - probably most - don't.
Personally, I think such programs are a nuisance and a waste of resources, at this time anyway.
(Unless you run Windows on your Mac, of course.)
If you've got a few hours free, read the dissertation by Linc Davis here:
I think it is pretty definitive.
You'll find that people who use AV programs have exactly the same experience as those that don't: their macs are not infected by viruses.
The only difference as far as I can see is that those who use them sometimes suffer from bugs, system slowdowns and - in the case of those that paid money - a lighter wallet.
I suspect you can figure out yourself which group you want to belong to.
Does anyone actually use an antivirus program for MacBook Pro Mountain Lion?
Here is what I consider to be an excellent article by Rich Mogull on that subject which was just published today Do You Need Mac Antivirus Software in 2013?
I have AV software installed, but do not have it configured for on-access scanning. I've done this for numerous Macs I've worked with, and I've never had any problems having the OS configured with these programs. Some of these programs are more intrusive than others (ie, they include firewalls, on-access scanners, and other active components that can destabilize the system if bugs are present), but most can be configured to have negligible (if any) impact on the system.
However, whether or not you need AV software is up to you and your computing practices. With proper observation of safe computing practices you can avoid most if not all malicious online activity, but for those who doubt themselves or who might be using their systems then it is one of several tools that can be used to help manage the potential of threats.
Granted the number of malware threats for OS X are minimal, but they are there and if you are at all worried about them then one option is to use these tools to periodically check your system. Keep in mind these tools are just options to help, and are only useful retroactively once malware threats have been identified and defined.
One last benefit of AV software is that it can help identify Windows malware, such that may be attached to junk mail and other messages in your inbox. Any of this malware wont hurt your Mac, but being present may be inadvertently forwarded to others with Windows PCs who may be harmed by it.