When beliefs develop and become understood in that way, there is always the danger that things will become part of the theology that are inimical to the gospel, sometimes deeply so. Perhaps the most obvious example is that of the beliefs that were common among Mormons prior to 1978 about the behavior of blacks in pre-earth life. Trying to justify the Church's exclusion of blacks from the priesthood, people invented offensive, racist beliefs, such as that black people were less valiant before this life than were others. These became church doctrine in the hermeneutic sense I have just described: they were commonly adopted as part of the larger explanatory narrative. Though never officially sanctioned or promulgated by the LDS Church, such beliefs were sufficiently common as to constitute Mormon belief. Indeed, they were sometimes taught by high-ranking church authorities as doctrinal. And they had effects on individual Mormons and Mormon communities that were, to say the least, harmful in their lives with black people.
Paul warns us not to be deceived by "philosophy," by which he means the wisdom of the world (Col. 2:8). Latter-day Saints often warn against "mingling the philosophies of men with scripture," which I take to be a variation of Paul's warning: we ought not to allow the world to determine our understanding of the gospel.
But the warning is in vain unless it is understood as an ideal toward which we strive. We cannot but understand things through the lenses of the time and place and culture in which we live. The gospel should offer us a new way of seeing and being, but in this life that new way can never be completely disentangled from things like language and culture.
In spite of that we must try. Formal or systematic theology has long been one way to try to keep the wisdom of the world from infecting the historical and hermeneutic development of belief. Tell us what the "pure" doctrine is and we will measure everything by that.
Many Mormons believe that there is such a pure doctrine somewhere, some set of propositions that lays out clearly all or most of what we believe and perhaps even why, and does so apart from historical or cultural influence. Unfortunately they too often think they found it in their BYU religion class or in a book by their favorite LDS author. Most BYU professors are not guilty of teaching as if they have an inside track to religious truth; many books by Mormons about Mormonism are quite helpful. Sometimes when there is a fault it lies in the student or reader as much or more than in the teacher or author.
Perhaps God could give us a list of the elements of pure doctrine, though whether he would is another question. After all, he is more interested in how we live our lives and keep our covenants than he is in our beliefs. In an LDS context, most often our human accounts of supposedly pure doctrine run exactly the same risks as do the beliefs that come about hermeneutically (which is no surprise since they too came about that way). They run the risk of harm created by beliefs that are inimical to the gospel. A formal theology can't solve the problem.
The ultimate solution to the problem, as the revelation of 1978 shows, is continuing revelation. We believe in continuing revelation not only to our prophet and the apostles, but to all other leaders and members: reliance on the Holy Spirit of Truth rather than on what we have decided is truth can help us keep from mingling the philosophies of men with scripture. And theology can help us with that.
As we think about our beliefs, we can use theology to help us see where the fractures or fault lines are, where we find things we do not understand as well as we assumed, where we may need to call on the Spirit more fervently rather than on the philosophies of BYU professors and others like us. Though theology can be a danger to those whose religion ultimately relies on revelation, it can also help avoid some of those same dangers.