I think class diagrams are a good way to visually display classes and hierarchy. If I have a package or module that has 10 classes and I want to show inheritance, properties, and methods I think it’s easier to show the class diagram then hand someone 10+ source files.

Also, class diagrams are more useful when you can’t give out source.

That being said, I think you can autogenerate class diagrams from code so it’s not like you should spend a lot of time making them.