The projects that is have worked on, the milestones were the body of the project. I think that missing milestones can result in failure of the project. At least in the projects that I have worked on all the milestones were extremely critical and were considered in the panning initiation and planning phase as critical. I do think that they can be less critical milestone like filing, meeting etc. that are not part of the projects major requirements. Some milestones may require the completion of other milestone and the project may not be able to advance until these milestones are meet.