I think you are dead-on when asking about the usefulness of the diagram. But you are asking the wrong people.
There are two groups of people who are much more qualified to answer your question than we are: those who will consume your use cases, and those who made up the template.
Knowing whether and why those diagrams are needed won't only help you to decide whether to produce them at all, but also make you more efficient in deciding how much effort to put into them, and more importantly, on which aspects to focus the effort.
See also
http://www.agilemodeling.com/principles.htm#ModelWithAPurpose
The soul is dyed the color of its thoughts. Think only on those things that are in line with your principles and can bear the light of day. The content of your character is your choice. Day by day, what you do is who you become. Your integrity is your destiny - it is the light that guides your way. - Heraclitus