Is the Doctrine of the Two Kingdoms the Historically Reformed Position?

In his essay titled "The Two Kingdoms Doctrine and the Relationship of Church and State in the Early Reformed Tradition" (Journal of Church and State, Autumn, 2007), David VanDrunen points to Calvin as advocating the position that has become known as the doctrine of the two kingdoms (hereafter 2K).

The Geneva Reformer explicitly distinguishes between what he calls the civil and spiritual kingdoms, highlighting three distinctions between them. First, the spiritual kingdom is redemptive in character while the civil kingdom is a realm of God’s providence rather than his redeeming grace. Second, the spiritual kingdom is heavenly while the civil kingdom is earthly. Lastly, Calvin argues that the spiritual kingdom finds its present expression exclusively in the church, while the civil kingdom expresses itself in such realms as government, science, and the arts.

VanDrunen then turns his attention to theologians from the heyday of Reformed orthodoxy. Rutherford, for example, distinguishes between "the kingdom that is of this world" that "fights with the sword," and "the kingdom that is not of this world" that "fights not with the sword." He calls the former "the magistrate’s kingdom," while referring to the latter as "the church and kingdom of Christ." Elsewhere, Rutherford speaks of "the kingdom of this world" and "Christ’s other kingdom, that is not of this world."

Turretin also follows Calvin in distinguishing the two kingdoms: "Before all things we must distinguish the twofold kingdom belonging to Christ; one natural or essential; the other mediatorial and economical." The "natural" kingdom, writes Turretin, is "over all creatures," while the "mediatorial" kingdom is "terminated specially on the church."

One can say what one will about the 2K doctrine, but he will be hard pressed to demonstrate that it was not held by Calvin and the seventeenth-century theologians who followed him. Look where you please, what you will find is a distinction between the sacred and the secular, between the civil and spiritual, and between cult and culture.

So I guess my question is, If the 2K position is thoroughly Reformed, why are those who hold it today looked upon with such suspicion?