Why are plain-English jury instructions important? To answer the question, let me introduce you to Arnold Kling. He's a smart guy — he has a Ph.D. in economics from MIT. This past winter, he was summoned for jury duty, and wrote about his experience. What do you suppose this MIT-educated Ph.D. thought of the jury instructions? In a phrase: "user unfriendly."
Although I thought that I had understood the judge's instructions as he was reading them, he offered to provide us with a written copy, and when we were given one copy there was sentiment that all of us should obtain copies, so that was done. We began our deliberations by going over the instructions, but any hopes that they would provide definitive guidance were unfounded.
In my view, the instructions ... were almost impossible to sort out. I could imagine a lot of jurors simply giving up on interpreting the law and instead making a judgment based entirely on "gut feel." That is, rather than making a logical determination, a juror might say, "I feel like it was murder" or "I feel like it wasn't murder." In our case, I think we tried very hard to understand and follow the law. But we also honored our gut feelings, and I believe that it was right to do so.
Let that last paragraph soak in. A juror as intelligent as any you'll encounter found the jury instructions "impossible to sort out." He suggests that as a result, the jury he served on gave up on trying to follow the instructions; instead the jurors followed their gut. The result in this case: a hung jury, with the criminal defendant, unable to post bond, spending several more months in jail awaiting retrial.
That's why jury instructions must be written in plain English. (For some tips on accomplishing that goal, read this article by Joe Kimble.)
(Thanks to Roger Shuy of the Language Log.)