Visualizing how the process you want to express in software works is a great technique for clarifying what you want to accomplish, but I'm skeptical that it is a good way to actual do programming. The simple reason why is that error conditions and corner cases are hard to express visually. That said, I think drawing out what you're trying to accomplish is helpful. It's basically a form of lightweight specification.
it is in fact very helpful to model out your error states, esp. if you can see it on a screen in some sort of flow. this is why people do modelling, to understand better what you want to code.
But you are right, there are many things that are difficult to express graphically (at the moment) but I think this can be solved in the near future
