Security of the shared secret key for a smart meter (Linky)
In addition to the other answer

Using asymmetric cryptography in the meters would have some benefits:


it can make passive eavesdropping of meter/server communication useless, even to a party holding or able to use the server's private key; something not achieved with secret-key cryptography. 
it can ensure that any central key leak can not compromise the capability to authenticate messages from the meters.


But that's about all I see. In particular, even a full blown asymmetric public key mechanism can not solve the most serious problem that if the central system is hacked, and the hacker can communicate with the meters as the central system does, then all the capabilities the central system has (including reading any meter, or/and making any parametrization of the meters) are available to the hacker. A complete hacking of the central system is thus a disaster, no matter what.

That's why such central systems should be designed with multiple layers of security. In particular, they should hold secret keys, and the server's private key if some asymmetric cryptography is used, within devices intended to never leak such secrets, like HSMs or Smart Cards. That way, penetration of the outer layer (e.g. server's OS) simply can't leak secret or private keys (only, at worst, allow their use). After an attack, restoring the integrity of the server can hopefully restore security and functionality (unless the hacker managed to change the highest-level individual meter keys).