One use case in my SharePoint demo environment is to show that certain web part pages have mandatory web parts while others can be customized by the end user. The way to accomplish this is to put the web parts in different zones. Then add the following properties to the <WebPartPages:WebPartZone> element: AllowCustomization="False" AllowPersonalization="False" AllowLayoutChange="False"
Simple! Or you could do this using SharePoint Designer.