Absolutely right. Something like this needs to be installed along the DMZ, or at least near Seoul and Inchon for their defense.
Right now, the Kim dynasty has the ability to make good on their threats to light Seoul up in flames at the click of a button. Only political will, not military capability, prevents that from happening — and with dictatorial leaders in Pyongyang of questionable sanity, the South Koreans would be crazy not to try to build an anti-missile defense system.
North Korea is a state actor, not a non-state terrorist group, but what is happening now in Israel with Iron Dome may be a perfect example of how high-tech countries can counter the inherent advantages of asymmetrical warfare. While a Hamas rocket may cost $1,000 compared to $60,000 for the anti-rocket missile, I think it's patently obvious who has more money available and more ability to install defense systems without being tracked by drones and having their operations blown up. A 60-to-1 cost ratio is not a problem for Israel versus Hamas.
In the Summer of 2011, according to Wiki, South Korea did express an interest.
“During a visit to Israel in the summer of 2011, Kwon Oh-bong, vice commissioner of the Defense Acquisition Program Administration in Seoul, expressed interest in purchasing the system in order to counter the threat posed by North Korean artillery, rockets, and missiles”