There can be a stage between "I have kind of an idea of what this is supposed to be" and "I'm ready to code this", where you think carefully about what this thing is actually supposed to be, and how it's supposed to behave and interact. It's not amiss to think for a bit before creating the code.
I'd rather spend some time making sure I'm building the right thing, rather than testing that what I built correctly does the wrong thing.
On the other hand, if you want to argue that UML is not the optimal way to do that, you could make a case. It makes you think through some questions, but those may not be the only questions, and there may be other ways of thinking through those areas than drawing diagrams.
And if you want to iterate your designs, UML is a painful way to do so. You'd want to design in some other medium that is easier to change. (Maybe something text based?) But if you're thinking through all the design issues in another medium, and iterating the design in that other medium, then why produce the UML at the end? To communicate the design to other people - that's the point of UML. But if you can communicate the design better using something else (like maybe the medium you actually design in), then why produce the UML?
I'd rather spend some time making sure I'm building the right thing, rather than testing that what I built correctly does the wrong thing.
On the other hand, if you want to argue that UML is not the optimal way to do that, you could make a case. It makes you think through some questions, but those may not be the only questions, and there may be other ways of thinking through those areas than drawing diagrams.
And if you want to iterate your designs, UML is a painful way to do so. You'd want to design in some other medium that is easier to change. (Maybe something text based?) But if you're thinking through all the design issues in another medium, and iterating the design in that other medium, then why produce the UML at the end? To communicate the design to other people - that's the point of UML. But if you can communicate the design better using something else (like maybe the medium you actually design in), then why produce the UML?