assuming its not spoofed connections we are talking about (in which case listing thousands of IPs is of course useless but those can probably be detected in an easier fashion...once again by the upstream ISPs who implement Unicast RPF that mitigates spoofing or do a backscatter analysis)
and of course......putting an ACL with thousands of entries in a firewall has its own complications
if it is a botnet running a script like you describe, most definitively it sticks like a sore thumb and can be easily blocked......simply using a packet inspector that matches the script in question
So Wolfie, do you believe that this is something like using a thousand zombie systems to simultaneously launch smurf attacks against a remote host ?
The major advantages to an attacker of using a DDOS attack are that multiple machines can generate more attack traffic than one machine, multiple attack machines are harder to turn off than one attack machine, and that the behavior of each attack machine can be stealthier, making it harder to track down and shut down.