A denomination can have an impact larger than the sum of the impacts of the individual churches. The Bible pictures financial and operational cooperation among New Testament churches, and virtually all churches cooperate with other churches in some fashion or other. Southern Baptists have merely formalized that spiritually-natural phenomenon, and God has blessed their blend of freedom and cooperation. Within the Body of Christ, there is a great diversity of gifts, temperament, taste, and experience. Churches benefit from this range of qualities within their own fellowship and across the denomination.
Churches learn from and complement each other.
This is not a matter of moral or doctrinal compromise. You cannot believe and do just anything and remain a part of the Southern Baptist fellowship. All Baptist bodies have limits. But within those limits, there is room for significant cooperative diversity. While there is a place for Biblical separation, a coming apart for the sake of holiness, separation can go to extremes.