Alexis Smirnov

In any case remember that use case diagrams are very close to useless. […] The diagram makes a visual table of contents, but nothing more. [Martin Fowler's Bliki]

This point from The Master of UML matches my experience. Apart the “visual table of contents”, the only other application of use-case diagram I’ve found useful is requirements triage. I’ve found it very productive to perform triage of requirements with domain experts (business folks) on the basis of use-case diagrams. When dropping or modifying a use case, the impact on end-users was immediately clear to everyone (including people in suits :) because it was easy to see who exactly is impacted by the changes.