What's the point of a Meet In The Middle attack while using, for example, a double AES encryption and using one time keys? You can recover the keys for a secret message already known and you can't use those keys to retrieve other secret messages. So why would an attacker use it?
As mikeazo points out, a meet-in-the-middle (or any other known plaintext attack) against AES is believed to be infeasible; I'll generalize your question to "if we never reuse a key, why do we care about known plaintext attacks? If the attacker already knew the message, why do we care if he could decrypt it?"
The reason is that the attacker might be able to use that attack even if he has partial information about the plaintext. For example, suppose that the attacker knew (or guessed) that the text of the message that Alice sent Bob starts off with:
"Hi, Bob! The password to my bank account is "
In that case, the attacker may be able to use the known partial plaintext to recover the key; if he is able to do that, he can then decrypt the rest of the text, including the bank password.