Other people in this thread have expressed it, but the reason it's not useful is because most developers are doing something like UML, but it's not UML - so you get none of the benefits of the Unified part.
When I see a good UML diagram I know immediately what the bit of modelled code does. When I see a weird, cobbled together diagram of bits of code the creator thinks defines the state, I have gained nothing of value from viewing the diagram.
In my experience, when I see a good UML diagram I know I am seeing a picture of how someone thought something was going to work before they (or more likely someone else) actually built it. It might or might not accurately reflect how the code actually works. In particular constraints the diagram claims exist may or may not actually be enforced in the code. The UML diagram says a Foo only exists for the lifetime of the Bar that references it; well, this memory dump from production says otherwise, so I don’t care what your diagram says, the code is what matters.
When I see a good UML diagram I know immediately what the bit of modelled code does. When I see a weird, cobbled together diagram of bits of code the creator thinks defines the state, I have gained nothing of value from viewing the diagram.