It would depend on the carrier. For travel on AA metal I think that you'll be fine - you'll be able to at least use US miles for either AA or US flights. Remember that most changes can't take effect till after the merger (looks like this August/September but it depends on regulator approval). So it might be wise to wait until after the merger to purchase tickets for fall 2014. Waiting is your friend - the longer you wait the more that will be known for sure.
Jim