Having worked with some of the first few Rational employees, my understanding is that Rational was/is very good at sales through their consulting arms (hired a consultant? congrats, you just paid a member of their sales force to wear your employee badge!). They don't have particularly great usage numbers on those sold products, though.

That said, there are some very useful UML diagrams for non-architect-titled folks. For people stuck using class-heavy systems (some of which are out of their control), the UML Sequence Diagram is both useful and a very compact way to explain "what happens when the user goes <poke>": http://en.wikipedia.org/wiki/Sequence_diagram