The problem is that not all new viruses will be caught if they are fast enough.    Here is a timeline of a virus' life.  

1. Virus released into the wild.  
2. Antivirus companies learn about the new virus.
3. Antivirus companies incorporate a signature for this virus into their pattern file for detection of the virus by their software.
4. The pattern file gets downloaded to your computer, and you are able to detect the virus.  

The problem is that between step 1 and step 4, the PC is vunerable, and the virus is able to spread to a machine even if it has virus protection software installed!!!!  

You may ask why does this matter?   I was administering an email server for a large company.   When the LoveLetter virus came out, I shut down the POP3 service on the mail server from the time I heard about the virus on the news until the time that the virus pattern file came out.   This meant that our people were not able to retrieve their email until the virus pattern file was released.   That equals a lot of angry people.   Once the pattern was released, I scanned all of the email files on the server, cleaned up the emails that were infected, and prevented the spread of the virus within our company.   We did end up having one of our servers infected, but that was because someone was retreiving personal email via an external mail account, and the virus was able to come in that way.   We could have prevented that infection by keeping the virus software current on all of the PCs as well as the servers.