SOT, As you say, we shall have to agree to disagree. But that is what being human is partly about; we all have differing backgrounds and experiences, differing understandings and perceptions. And, we can continue to be forum 'friends', who are sufficiently adult and mature to enter into strong discussion.
There is nothing wrong with each party holding strong views and opinions; in fact, one of the major problems today is, that political correctness insists on shutting down open discussion, and enforcing control over free thought and free speech. Hence, very little real learning can take place.
We can all have differing opinions, but only through dialogue can our opinions be tested, confirmed, or even changed. Many of my earlier opinions have changed over time; for example, I once believed in evolution. Now the theories of evolution appear to me be the product of insane thought and wilful blindness.
My primary reason for disagreeing with secret courts is: Secrecy behind closed doors places unlimited trust in those who are sitting in court making all the decisions, and sitting in judgement over an accused, without any monitoring or referring to the perceptions of 12 independent jurors. Unlimited trust in fallible humans, even those of high rank, provides no guarantee of justice; even court officials, and judges, can be corrupt, or just mistaken. Justice that isn't seen to be just, cannot be relied upon, or trusted to be just.
We've recently had the High Court sitting in judgement over the Brexit situation. Even these high Lords of the land can't all agree on what is the correct interpretation of the law. If law were so precise, so clearly understood and interpreted so as to cause a number of eminent judges to all agree all the time, on every occasion, then there may be a remote argument for secret courts; but that patently is not the case with judges (or anybody else) and law. Even Prosecuting Council, and Defending Council, will see differing interpretations of the same law, which is one reason that cases go to open court before a jury.
There is also another little understood, and vitally important power that juries possess. Juries have the power to declare a law unjust. Even the judge has to give way to the jury. Juries have the power to set precedence.