Why should a jury decision be unanimous or not?
A jury decision has to be unanimous in criminal cases only, not civil cases. In a criminal case, depending on the charge, we are talking about the ability of the State to take away someone's freedom or even to put them to death. This means the burden of proof is on the State.
Proving their case beyond a reasonable doubt, then, is the threshold, and this is why criminal trials should reach a unanimous verdict. Many times, including on juries I have been on, the accused was most likely guilty, but the case is weak or the evidence lacking, and the system, as we have designed it, protects those we cannot prove are guilty, even if they might be.
I like this protection. Juries are made up of human beings and human beings are fallible and act emotionally. They are imperfect, and the only guard I have against this imperfection is that all 12 jurors be convinced of guilt or innocence in order for a verdict to be determined.
On the other hand, this is a very inefficient system. Twelve impartial jurors are hard to find in many cases, and there is a significant cost to society when these juries cannot reach a unanimous verdict, as they have to be tried again at additional cost, or let go to potentially commit crimes against society once again.