"In practice it's not clear if Apple devices encrypt to this key directly or if they engage in an OTR -like key exchange protocol. What is clear is that iMessage does not include a 'key fingerprint' or any means for users to verify key authenticity, which means fundamentally you have to trust Apple to guarantee the authenticity of your keys. Moreover iMessage allows you to send messages to offline users. It's not clear how this would work with OTR."

So I guess the need to send messages to offline users ruled out the use of OTR.