It's an interesting question, Greg. IMO, the reason for using a modal dialog should be to focus the user's attention on completing the task at hand before doing something else in the UI. So in working from that premise, it begs the question of why someone needs the dialog to be movable. If the concern is that it's covering content on the parent screen that the user needs in completing the task, then it seems like a modal is either the wrong solution or that information should be available from within the dialog.
I'm also interested in what others think about this. I wouldn't be in favor of making PatternFly modals movable unless there is a use case justification that we can point to.