even if you crypt packets sent from client to server or server to client and even if it has a good hack-proof, it wont mean much for being hack-resistant. MMO cheaters generaly modify data before they sent, using some memory tools, or they dont do anything with data, they use BOT programs which plays for player.
In this type of case, encryption of packets wont stop them, you need to use something like x-trap or hackshield.
Youre right thats true in most cases, but if the traffic can be manipulated easiliy - there are much worse things possible e.g. buffer overruns
