Do you believe that project management diagrams (UML, Gantt chart, WBS, etc.) are useful tools? Do you make use of them in your professional life?

Yes they do. But, it really depends on the size and nature of the projects to use these tools. 

Yes these are very useful tools. We use http://www.majicdoor.com/ tool for project management which includes WBS, Gantt chart etc. 

We use EA (enterprise Architect) tool for UML diagrams and documentations (Both behavioral and Structural)

Selection of UML diagram types depend on the size and nature of the project

sure, to calculate the time required for each and every particular activity they are useful. 

Yes sure, i usually use these diagrams when I have to summarize the achieved and the upcoming activities for project steering committee

Sure.

of cousre wbs or work breakdown structuer that help us to decompose the scope in to work package that consider the lowest level of the wbs 

Gantte chart it is  agood tools that help us demonstrat our schadual acitivities with more details