Doesn't help against a common attack possible for all reasonably modern luxury cars (which happen also to be the most interesting targets).
For these cars it's enough that the key is near and it is considered a feature that no user interaction (like pressing a button on the keyfob) is required. This can be exploited by relaying the signal from the original key to an attacker who is near the car. Cryptography alone cannot protect against this attack scenario (which is called a "relay attack", not to be confused with a "replay attack").
