Most people's passwords are in the category of "moderately weak, but not necessarily leaked to the public". If you are to accept passwords at all, you are respo

Most people's passwords are in the category of "moderately weak, but not necessarily leaked to the public". If you are to accept passwords at all, you are responsible for protecting these users who have an expectation that your service is no worse than any other service's ability to protect passwords. There is no way surefire to force users to pick a truly entropic password. So an adequate amount of key stretching is not optional. In a way, adequate password stretching helps to treat passwords as the toxic sludge they are. The goal is to store them in the most irrecoverable possible format, regardless of the poor decisions of the users who entered those passwords, so that you as the service don't end up being the one making the problem worse.