That is a common problem in the Windows world: file types are normally indicated by file extensions, so it seems a good idea to tell the antivirus to ignore some extensions. It can indeed speed things but think of that scenario:
- on a machine I ask AV not to scan txt files because I know that text files do not contain viruses
- an evil person gives me a USB key containing a bunch of jpeg files and lists of those files with comments in txt files - one of those txt file is is fact an executable file containing a virus but renamed to txt
- I copy the content of the key to my disk - AV does not detect anything
- => a copy of the virus is now on my disk
A slighter worse one would be if the key contained a clean version of an installable application for browsing the files, and if the owner of the key explains that the installation is awfully complex and breaks when an anti-virus is on (think of some Pinnacle products...) but he put a batch file that automates it - in the middle of the batch file, he renames the false text file and executes it...
In the above example, replace txt with vmdk, and you will understand why rules stopping the AV to ignore known extension actually lower the security level.
When possible, I do prefere to tell the AV to ignore specific folders, or even better specific files