Being in Oneworld wasn't exactly a huge boon last time they were in it.
You can't compare Oneworld then to today.
Back in 2007 there was no anti-trust immunity with AA and very limited co-operation in terms if codesharing.
Being in the AA/BA joint venture will allow for full codesharing across AA's domestic network (much expanded post the merger with US Airways, full FFP reciprocal recognition, joint scheduling and access to AA and BA corporate accounts.