I just read this answer to a popular question here, where it explains why lawyers aren't allowed in juries most of the time; here's an excerpt:
If a lawyer is in the jury, that person will undoubtedly be able to explain exactly what the law requires for a finding, or exactly why a certain finding should be had. They will advocate one way or another; this is undeniable. The instructions are purposefully confusing. The reason is this: when we fight over jury instruction, inherently, one of us will want an instruction that is hard to understand, for a lay person. This is because we want them to apply the law as it is commonly (mis)understood, not as it truly is, because that's not good for our case.
And a comment sums the answer up nicely:
tl;dr jury trials have become a theater where attorneys on each side will try to confuse the poor laypersons on the jury into voting in their favor. Lawyers will typically see through that and negate all the showmanship that the attorneys will do.
This is the first I've heard of this, and it seems unethical, unjust, and deceitful, all words that one would expect never to be able to use when talking about the legal system.
Why is this allowed? Why doesn't the law ensure that jurors, laymen and experts alike, are given clear and concise instructions?