I can't see AA hooking up with US because US, like TWA, brings very little to the table. The only valuable parts of US, as far as AA would be concerned, would be the shuttle and maybe the DCA and LGA slots. AA doesn't need a hub in PHL because they have JFK; and the trouble in the banking sector makes CLT look less attractive which would remind AA of RDU. Add to that the totally different fleet types and employee integration issues and one could see why it would be a totally foolish decision on AA's part to enter into such a transaction. AA would only want the shuttle slots and LGA and DCA slots; no planes, no people.
Like gaucho99 said, AA benefits from buying US by cherry picking her assets and throwing the rest away. AA and US have a lot of overlap in the Caribbean. AA would gain pricing power in that market. They could probably find a use for either CLT or PHL.