One photograph of a person crying or upset could be used to represent then entirety of a trial. Imagine if that one moment is published in a newspaper. The person would be immediately assumed guilty in the court of public opinion.
After about 20 examples of famously (to different degrees) guilty criminals it followed up with another 10 or so where the defendant was later cleared of the crime when new evidence came out, but still they sat "emotionless" as they were found guilty of a crime they didn't commit.
When you're sketching, you have to make choices about what's important and what to leave out. That means you have to study the scene pretty closely and pick what to represent. If courtroom sketching works like that, then I could see how having a skilled observer picking out important things to represent could be beneficial. If a particular moment is especially memorable, the sketch might capture and emphasize the drama in a way that a photo wouldn't. A sketch is kind of like the executive summary of a scene, removing noise and focusing in on what seemed important at the time.
http://static5.businessinsider.com/image/5273e8bc69bedd1614a...
UK court artists are not allowed to sketch in court. They must observe and then leave to sketch from memory (a fact that I was not previously aware of).