Short answer: It is almost certainly unnecessary.
Long answer: Mac computers are based on an underlying system called FreeBSD, which is similar in many respects to linux. This type of system is designed to be quite secure, and is very specific about permissions (which is why the computer asks you for an administrator password before changing certain system settings, installing software, etc.).
In general, the worst that a Mac virus could do without the user entering their administrator password is that it could delete that particular user's documents, files, pictures, etc. The core system should remain unaffected.
There may be exceptions to this, of course, but there are effectively no Mac viruses in the wild. There are thousands (tens of thousands?) of Windows viruses in the wild.
Running anti-virus software incurs a bit of overhead (system resources must be reserved for scanning files with the anti-virus program), and considering the very low risk of Mac viruses, I would recommend that it's not necessary to incur the cost of such software.