There is actually nowhere in the PMBOK which forces waterfall processes. I found that scrum fits perfectly as a lean implementation of large parts of the PMBoK, including the dreaded Earned-Value technique : the burndown chart is an implementation of that under the assumptions made by scrum (and turned upside down).
One problem with scrum is that it makes a LOT of assumptions, all of which are not always present. It happily ignores getting funding for your project for example.
Also not all projects are best done using agile methodologies, when your team members have somthing like less that 10% of their time for your project for example, or when you are faced with 12 week hardware delivery schedules. Also not all teams in an organisation want to work agile, but still need to work together, in those cases it pays to at least be able to communicate in a lingua franca and report your progress as the stakeholders expect it.
Well my point is that it is useful to have more tools in your toolbelt and a PMP comes with well filled toolbelt which complements the SCRUM tools very well.