I have to support that. ScanMail is awesome, I deployed it for 400 users worldwide and very happy about this decision. People don't swap floppies any more. If you are confident that every single workstation on your network is properly scanning for viruses all the time, then you have no need for an Exchange-based scanner. If you buy and properly install ScanMail for Microsoft Exchange from Trend Micro, I think you'll be able to do an excellent job of stopping a spread of viruses.
Email is one of the primary potential sources of virus propagation.
We have seen ScanMail for Exchange intercept and notify the sender, intended recipient, and the admin about email-borne viruses quite a few times in the past year. I would prefer NOT to have an infected attachment sitting in my Exchange stores, given the choice. Especially Word and Excel macro viruses. And these type of files are passed around almost exclusively via email. If they are sending infected files, I have the desktop group check their PC for proper antivirus setup.