Preference dialogs should be made Transient, but not Modal. When a user makes a change in a preference dialog, the change should be immediately visible in the UI. If the dialog is modal, the user may be blocked from seeing (and especially from interacting with) the change. This means they will have to close the dialog, evaluate the change, then possibly re-open the dialog. By making the dialog transient, we keep the dialog on top for easy access, but we also let the user evaluate and possibly revert the change without having to close and re-open the preference dialog.