Originally Posted by
dtremit
Not combining mileage would save them negligible money, and would create immense ill will and bad press. That's why every merged carrier has used combined totals, to my knowledge -- including US following the HP merger, and AA following the TW purchase.
They'll certainly eventually combine redeemable miles, but for the short-term, my guess (having worked on enterprise systems) is that from an implementation standpoint, this was the easiest to do. They already have framework for crediting to/from other carriers and giving benefits to their elites, and could fill in the pieces and data necessary to work with the other half of new AA.
Also, some pieces of the equation (e.g., EQP on AA) may not map nicely to data on the other half's backend.