I believe that AA is much more restrictive than their biggest competitors UA or DL, especially if you are elite. The other two allow you to change routing as long as it's the same origin and destination. UA even lets you change from a connection to a nonstop, DL has it free for DL gold (AA PLT equivalent), and UA has it free for all Premier members.

Even overlooking the inability to switch to a nonstop, it's often best to change hubs like BOS-PHL-MSY to BOS-CLT-MSY or v.v. because the earlier flights through a different hub is better timed than the earlier flights through the hub you were in, or there's better weather/operations that day in a different hub.