Scope creep is one of the biggest issues concerning project management, especially IT projects. It results in adding too many features to a project and ultimately leads to going over budget and missing milestone time requirements. It is the project managers job to determine if scope creep has occurred in a project and make the appropriate actions. In this case it is entirely appropriate to cut non-critical features and get the project back on track for completion.
However, simply sticking to the budget is not always the best course of action to take. In many cases, milestone requirements may have been improperly calculated based on early misunderstandings of the technical aspects of the project. If a feature is critical to the project, cutting it out simply isnt an option. A project manager needs to know what functionality is critical and what is not. If a critical feature deadline needs to be extended, then that is what needs to be done and the project manager will have to deal with the consequences.