I would actually go the other way and say there should be 100+ ranks. A typical use case for this tag would be to show important features on maps at lower zoom levels than their tags would otherwise suggest. So if I'm looking at a general-purpose map of Chicago I would want every building and every sculpture labeled, but I would expect that the bean and the sears tower would be labeled. But when the labels collide I would expect the sears tower label to be prioritized because it is a more prominent landmark. And this has to scale to every zoom level. Having a wide range of ranks allows for more granularity. The exact differences don't have to be defined, just the relative importance between features.