Effectiveness
Although many Operating Systems are now immune from being used in this attack, Some popular systems are still vulnerable; making the idle scan still very effective. Once a successful scan is completed there is no trace of the attacker's IP address on the target's firewall or Intrusion-detection system log. Another useful possibility is the chance of by-passing a firewall because you are scanning the target from the zombie's computer, which might have extra rights than the attacker's.
Read more about this topic: Idle Scan