To have some UML diagrams for documentation reasons is definitely a very good point! It would have been helpful in many, many situations to have at least some small UML diagrams to get a brief overview of some complex parts of an application.
From your answer I guess you have quite a bit practical experience with this. So just out of curiosity: How do you handle bigger projects with 100+++ classes? Do you break everything down into modules or are some tools good enough to help you manage diagrams of that size? My personal experience is that UML diagrams are most helpful to visualize details of a smaller part or module of an entire application (mostly with class diagrams) but I can hardly imagine to model a really big project with them.
Which of all the available types of UML diagrams do make sense for practical use in bigger projects in your opinion?
And if you say UML is most helpful for the transition from design to code then how do you handle iterative changes which you obviously will have if you don't use a simple waterfall model as development process?