The immanent Trinity/economic Trinity equation sets up a very traditional Roman Catholic mutualism. It makes God dependent on creation, which is one of the main problems with Aquinas, though he would probably deny that such was an implication of his theology. What God does in history that appears to change reflects only the change of the creation/created being, not a change in God. God is always consistent in His treatment of creation, so if the creation changes, then God's treatment of that person will change without reflecting any change in God Himself. But if you equate the immanent with the economic, then that forces a change within God Himself if there is a change in the economic Trinity. Ultimately, then, the equation of immanent and economic is equivalent to a denial of the immutability of God. The Reformed Forum guys are really great at filling out this discussion. In fact, they thread the needle between the very real problems of Oliphint's treatment of Aquinas, and the Reformed Thomists.