The following question is the converse of the implications of a "PLA missile defense system." What about the American missile defense system? The points raised apply equally to the PLA missile defense system.
Should China worry about U.S. National Missile Defense (i.e. NMD)?
1) Firstly, NMD doesn't work. There was a recent test failure (see Despite Failed Test, Missile Defense Program Must Continue | The Foundry: Conservative Policy News.
2) Secondly, there is no way of knowing whether the entire system will work during an actual war. There is no real confidence in the NMD system because you can't conduct a real test of hundreds of incoming MIRVed missiles that releases thousands of nuclear warheads.
3) Thirdly, if you detonate a nuclear weapon high above the atmosphere, it will create an EMP to permit the penetration of follow-up attack missiles.
4) Fourthly, China can use nuclear IRBMs to wipe out the American interceptor base in Alaska. Once the NMD missile base has been destroyed, the shield is gone.
5) Fifthly, unless the American interceptors can destroy the incoming missiles in the boost or mid-course phase, no current technology can stop incoming nuclear warheads traveling at Mach 10.
6) Sixthly, MIRVed ICBMs will overwhelm any realistic NMD. For example, twenty ICBMs with 10 MIRVs = 200 warheads. The defender is at a severe disadvantage. The attacker can always build more cheaper MIRVed ICBMs than the defender can build an interceptor rocket for each incoming warhead.
7) Seventhly, China is building her own NMD. You have a shield. I have a shield. Now, we're even.