While it is clearly undesirable to have any malware files on a computer, the risk of Windows malware being accidentally or inadvertently transferred to a Windows computer is fairly low. For example, how often do you forward a spam message to someone else?
It's not like the days when macro viruses affecting Microsoft Office were commonplace. Even though their payloads rarely worked on a Mac, they were able to infect template files and hence new documents, and it is common practice to send files to other users.
Also, there would be very few Mac users that run Windows from a Boot Camp partition or in a virtual machine without running a Windows security product.
But that 2.7% incidence of Mac malware suggests the problem may be bigger than some users would like to admit. However, it is possible that the sample overestimates the overall situation as it seems plausible that people who have some reason to believe their Mac may have been infected are more likely to install a product such as Sophos's.
When Sophos Anti-Virus and other products are available to home users at no charge and have so little effect on performance, it's hard to see why they aren't installed on more Macs. Even though security companies aren't always as quick to detect new malware as they would like (eg, the Sabpab variant that exploited a Microsoft Word vulnerability), they did provide protection against Flashback.K - which reportedly infected more than 650,000 Macs - before Apple patched the Java vulnerability that it exploited.