It seems insecure to leave another point of access open to the conversation, if merely out of principle, due to concerns over leaving a window open for eavesdropping.
There's no reason that has to be insecure (depending on your definition of "secure"). There's also no reason the user shouldn't be allowed to make that choice for himself.
If you're lucky the 3 clients will negotiate down to disabling the encryption. But normally they'll just engage in a mutual, infinite re-send loop...