Yes, the Old Testament has to be included in the Bible.
It is likely that people who say that it should not be included are taking the New Testament in an Old Testament way. I mean they replace the commandments of the law of Moses with the new commandments of the New Testament.
In either case people tend to view religion as basically a list of Dos and Don'ts. With the Old Testament you had the ten Dos and Don'ts. Now they assume that these have now been replaced by the Dos and Don'ts preached by Jesus and the apostles. If you just do the list of things you're suppose to do then you are a Christian.
So, they may surmise "Why do we need any longer the old list of things to do? Now the teaching of Jesus has a new list of things to do. So the Old Testament is really not needed."
It could be that this "Obey the List of Things to Do" mentality sees little need for the old Do List to be around as long as the new Do List is here.
The new Do List teaches us to do such things as hate your self, or turn your cheek when struck, or give to the poor, or care for your family. Many things seem pretty similiar to the Old Do List. But many things seem contradictory to other Dos in the new Do List.
Actually, the New Testament is about abiding in, remaining in, dwelling in, and walking in the realm and sphere of a living Person - Jesus Christ. God, in the New Testament, is in Christ, as the Holy Spirit, imparted into the innermost being of the Christian disciple.
"But there are many commandments in the New Testament. Are there not?" some would say. My answer is yes there are.
So what gives then? That's all I'll write for this post.