> the diagram becomes far easier than a text description
If the text is hyperlinked it can be every bit as easy to follow as a diagram. In fact, it can be easier if things are really complicated because it allows you to focus your attention more easily just on the parts that matter to you at any given time.
I don't agree with this at all. People naturally draw diagrams to demonstrate complex things. Any time a technical discussion gets complex, someone walks to a whiteboard or pulls out a piece of paper and starts drawing flows or interactions or whatever to clarify the discussion. Never have I seen someone say, "this is getting hard to follow, let me write some prose".
If class diagrams are useless then so are bar graphs and scatter plots. You can just list out the data points and everyone can read them.
No, graphs and scatter plots are useful because the spatial distribution of markings actually conveys useful information: things that are physically close to each other are mathematically close to each other. But most UML diagrams aren't like that. And even for the ones that do try to convey information that has spatial analogs (like containment) UML doesn't actually take advantage of spatial relationships between elements. For example, the natural way to express containment is by putting one element inside the other in the diagram. That makes it obvious what contains what. But that's not how UML does it.
Just about the only diagram that has a natural mapping from spatial relationships of the diagram elements to the actual underlying semantics is the sequence diagram.
"Spatial" isn't particularly meaningful when talking about classes, so it's not surprising that UML doesn't generally attempt to express that.
We can probably just agree to disagree here. I think class diagrams are often quite useful. They existed before UML and they are used even by people who don't use UML.
If the text is hyperlinked it can be every bit as easy to follow as a diagram. In fact, it can be easier if things are really complicated because it allows you to focus your attention more easily just on the parts that matter to you at any given time.