We need distinctively to have the purposes that Jesus Christ gave us. We need distinctively to be God's family. We need to establish and preserve that distinctiveness. However, in our desire to preserve the distinctiveness of the church we have reasoned ourselves to a set of different concerns. Cooperative efforts endanger our distinctiveness.
Cooperative efforts run the risk of compromise.
Cooperative efforts might appear to endorse error. Personally, I am very familiar with this line of reasoning because, in the past, I held that perspective strongly. That is a fair question. It was not a quick or sudden transition, but a slow process that took several years. The process began during my mission work in Africa.