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A good article over at Techrepublic focuses on the narrowing bandwidth crunch many businesses are feeling:

As e-mail worms like the resilient Sober variants spread, they consume a lot of bandwidth and computing resources. However, it really doesn't take much worm traffic to disable e-mail services for an entire company, especially if that organization is using a single T1 line. Find out why Jonathan Yarden says the days of relying on a corporate T1 connection are over.

This is definitely true -- many of CentreSource's infrastructure clients here in Nashville have a T1 or sometimes less -- 512Kbps, 768Kbps. Even smaller networks are starting to put these connections to the test, and an e-mail worm running rampant can quickly cripple the connection:

By this point, you would think that e-mail worms would be a thing of the past, but they only seem to be getting worse. One reason for this is because more and more users are forgetting the first rule of using the Internet: Keep yourself protected and secure—because you really have no other choice. And that includes not opening suspicious e-mail attachments or running programs from e-mail.

This is why a relay service like Swirbo can be particularly valuable -- not only does it catch these viruses (and spam), preventing both nuisance and harm, it prevents them from clogging your bandwidth. With mere desktop/server antivirus alone, you are protected from harm, but your internet connection will be affected for the duration of the outbreak.