From the Da Vinci HRex call, the indication is that member id can remain consistent across multiple coverages.  If that's true, then putting the member id in a field that's defined as "A unique identifier assigned to this coverage" is incorrect.  The identity of the coverage and the identity of the beneficiary as assigned by the payer are different things.  We can't count on the payer-assigned member/beneficiary id being present in Patient.  Therefore, we probably want member id/beneficiary id as a distinct element on Coverage.