Stopping worms in their tracks…

I was flipping through the Cisco Online IOS Documentation the other day and was suprised to learn that the effects of worms such as Code Red can be defeated at the router level by adding a simple filter (script). This has the effect of stopping such agents from chewing up bandwidth. If this can be done, why are we not hearing about it from the “Experts”? Is there a downside to this that makes it an undesirable option? It seems to me that the router would be an ideal point of engagement against net-bourne worms and trojans since everything must pass through a router to get anywhere. What do you think?