Actual Worst Cases

Easy to build worms
Steal an exploit, load into kit
Blaster (2003)? Deloder (2003)? Slapper variants (2002)
DDoS payload propagation
Damage to target, infrastructure
Blaster (2003), Deloder (2003)
Continued worms
Waste network resources
Compromised hosts announcing themselves
Nimda and Code Red (2001), Blaster (2003)