Why an anti-virus plug-in?

Suppose you get a new virus that your virus scanner doesn't recognize.
Your message gets dropped in your message base and you update your
virus scanner.
Next thing your virus scanner recognises the virus and it blocks
access to your message base as it sees that it is infected, but can't
pinpoint the infected message. Here your plug-in comes in handy, it
points you to the infected message, you delete it and you're done.

Other example. You download a virus infected message. Your virus
scanner blocks access to that file while in %temp%. TB only sees it's
unable to collect the message, doesn't delete it on server and tries
again next time. Virus scanner blocks the download and TB only sees a
failed download and tries again next time. Etc.
Because the whole procedure isn't handled properly, it's likely that
not only the infected message is downloaded over and over again, but
also the messages in the queue before the infected message and you'll
never see the message further on in the queue until you got rid of the
infected message via the dispatcher.

I don't even talk about encrypted connections or message bases.

Yes, the plug-in uses the scan engine of the major antivirus application, so they have to be the same brand.