Here is a great article on the new meta tag recently embraced by Google that was originally created by MSN.
Why do want to stop Google from using your listing in the Open Directory Project (ODP) aka DMOZ? Simply ODP has always been slow to update listings to reflect changes within company offerings or they might simply have it wrong. Either way, this could effect your ranking in the SERPs or even what is display as a site description within the SERP.
It's all covered in the article from Stepforth.