Imagine this conversation:
Wife: I was planning our son’s birthday party on Saturday. Is that a good day?
Husband: That works for me. I will be there.
Wife: You will be there? You are omniscient!
Husband: What are you talking about?
Wife: You said you would be there. That day is a week from now, and for you to know that you will be there means you must know all events: past, present and future.
Husband: No. I really don’t know all the future. But I am definitely going to be there.
Wife: How do you know you are going to be there if you are not omniscient?
Husband: Because I have a car, and I will just drive there. I have nothing else going on that day.
Wife: But what if you get hit by a bus? You cannot say you will be there.
Husband: Well, I guess I cannot say that I am “definitely” going to be there, in the sense that nothing ever can change the outcome. But those things are highly improbable, so yeah, I will “definitely” be there in the sense that barring any unlikely circumstance I will be there.
Wife: I do not believe you.
Husband: What are you talking about?
Wife: You don’t know the future, so how can I trust a word you say?
Husband: What are you talking about?
Wife: If you do not know the future that means anything can happen. When you say you will be there, you could just change your mind.
Husband: But haven’t I always done what I said I was going to do? You know me. I always go to our children’s birthday parties.
Wife: Well, if you do not know the future then you might go crazy and change. Because you do not know the future, because the future is not set, I cannot trust a word you say.
Husband: *confused look* … alrighty… I am going to go play with the kids now.
Most people would correctly identify the wife as being very low in stability. If her husband has proven to be reliable in the past concerning events, she is amiss not to trust his predictions of the future. After all, his character is known and he has the power to make his predictions a reality.
This scene, although a work of fiction, describes several debates between Open Theists and Calvinists. Calvinists instantly act like the wife in the above storyline. If “God does not know the future we cannot trust Him”. Here is Samuel Lamerson in a debate on theologyonline:
I am not sure that I would trust my money to an earthly gambler, and sure that I would not trust my salvation to a God who creates with no idea of what the agents of his creation will do.
This is echoed by Gene Cook in a 2007 debate:
[Paraphrasing Lamerson] “How can we trust Him if the future is open?” I agree: how can we trust him. And the response of Bob Enyart is, well, we can trust Him because God is loving, and God is good, and God is righteous. Bob, how do we know God is good, God is loving, and God is righteous? How do we know He is going to be good, righteous and loving tomorrow?… If you say that God is changing, how do we know He is not going to change his decision to accept me as one of His sons?
The Calvinist, to function in society, has a very low burden of trust for fellow human beings. What Calvinist will say they “do not trust” their wife because she has the ability to change?
But when God is brought into the equation, Calvinists discard all signs of rational thinking. This follows a long line of Calvinists trying to ignore how rational people converse, act, and think, opting instead for arbitrary and unreasonable standards. If God does not know something with 100% certainty, God is said not to know it. If God says He will accomplish something, it is assumed that God can only know it if God knew the future. If God is said to be sovereign that means God controls all things. If God is said not to know the future, we then cannot believe anything He says. This is unnatural to how people naturally function.
The really funny thing is that when God has to defend Himself against His critics, God gives reasons. Open Theists do not really have to work to defend these points against Calvinists. God defends Himself against those who think that God cannot know the future. In Isaiah 40-48, the message is echoed: “God knows the future because God is powerful and can bring about His purposes”:
Isa 48:3 “I have declared the former things from the beginning; They went forth from My mouth, and I caused them to hear it. Suddenly I did them, and they came to pass.
When God explains to people how He knows the future, it is not Calvinism. God explains that He knows things because He can do them. God does not rely on the irrational statement that people should trust God because God does not change. That statement is only found in Calvinist apologetics.
God is Open 
I have to say that I have seen more Arminian, or Open Theists as you call them, use this argument to make their point rather than a Calvinist. Calvinists do not believe in God’s sovereignty based on his omniscience and for-knowledge. That is an arminian based belief. I could have missed it and that may have been the point you were trying to make. I believe there comes a problem when the two discuss because one side wants to use philosophical argument and the other a biblical argument. Typically the Arminian is the philosophical thinker while the calvinist is the latter, I would say that Isaiah 48:3 is a verse showing a more calvinistic belief. God is completely and utterly sovereign. A Calvinist would believe God is omniscient, but they would also believe God is omnipotent. This is the most crucial part of the belief. If you are all-knowing and all-powerful everything goes according to your plan. For instance, God places people in their lives where they may or may not hear the Gospel, this alone shows God’s omnipotence and his omniscience, If God place a baby in an area where they will never receive the Gospel, isn’t that God using his power to create an elect? If you object to this then I ask, In the Old testament why is it, that of all the people in the world God chose Israel? Why not the Philistines? What about that young man God created and placed in the philistine culture of paganism, KNOWING that the Israelites would crush him in battle. Anyway, my comment could be completely off of the angle you were going for or event he point you were making. Just wanted to join. God Bless.