Not sure on how good AV protection on MAC, not seen any benchmark on it yet..
However there report worm/virus targeting MAC OSX.. in the news
Anything made by human is hackable by human
and if we think we're smart there other people which smarter than us..
Personally I would agree with you, and not 'them'.
You need to make a risk assessment on the cost to your company of cleaning up an outbreak or handling a data breach in the event these unprotected Mac machines were an infection vector.
I think once you work out the maths the numbers will speak for themselves.
if it helps, have a read of some recent McAfee Labs Blogs with the OS-X tag:
I'm sure you can find similar stories on any of the major av-vendor's websites.
Without any doubt, I would strongly advise against leaving any systems unprotected in the workplace. While the risk to Macs may not be as great as the risk to Windows PCs, there is a risk nevertheless. Even a "small" breach in the environment described could have major consequences.
Macs have also been known to accomodate Windows based malware. This could easily be transferred to Windows PCs through file sharing, USB devices, server storage or network shares.
There are less Virus or malware reported for MAC however.. if we look on Mac Malware closely MAC is not 100% imune to it..
check https://blog.avast.com/2011/05/20/mac-malware-%E2%80%93-a-short-history/ for reading..
Not only that since Windows Virus can not be run on MAC the higher changes MAC to be carier is also higher..
example people may not aware that their usb infected then copy over their file to MAC later these files copied over windows machine and these virus or malware triger..