Public Key is rarely if ever used for encrypting the messages, because it is many times slower than normal encryption. Instead, the public key is used to share the actual encryption key that's used. Unique session key doesn't require constant handshaking, because you're sending the public unique constant along with the message. if the secret key is "password", you can use the hash output of $md5(password sequential) as your session key, then send the 'sequential' along with your encrypted message. The receiver already knows the secret value, and uses $md5 to re-create the unique session key. Since the $md5 outputs are all different, this causes the same message to be encrypted differently each time. The secret password only needs to be handshaked the one time.
I agree that 1 strong cipher in a strong system is worth more than any group of ciphers that don't have both. The AES cipher is widely used, but the criteria for it included many things other than being a strong cipher, such as being able to be done in low-tech smartcards, short key setup times, low memory usage, etc. I'm not sure whether it would overwhelm mIRC to have to perform the 4kb encryption key-setup for each message in a busy #channel.
