Rationale of "r" AES key use in OTR version 3 AKE protocol?
The usage of the r key forces both parties to "fix" the public DH keys.

So Alice doesn't know Bob's public DH key before she's generating her own one. 

And Bob can not make the choice of the public key dependant on Alice's choice and vice versa.

This forces both parties to be honest and to generate both public keys at random as there is no opportunity to force a specific derived key that may have some weak properties.