The reason for splitting out the upref package instead of automatically incorporating it in the amsart and amsbook classes is this: It involves low-level surgery on an important command. This means that if ever this command changes in the future (as it did between versions 2.09 and 2e of ) we have a maintenance problem. And the benefit that upref provides is something that most users don't care much about. It can be used for in-house AMS production but it need not be inflicted on all users. Instead we leave the choice to the individual user.