Logic is a good thing, but the logical process does not necessarily lead to truth. If you do not start with the right premises, you do not get the right conclusion, logically enough. "Garbage in, garbage out" as they say in my line of work. If you want your logical results to be true, your starting point must be true.
"I am the truth." -- JesusJesus' saying does not compute in most syllogisms about knowing God. In fact, it is opposed to our natural way of thinking about truth. The truth is the sum total of what can be known, the highest transcendent perception of reality. The truth is where all decent thinking must begin and towards which all decent thinking must aim.
Jesus challenges us to understand God through him, to begin our systematic theologies with him, to start with him as our premise and end with him as our aim. Our natural thinking hardly knows where to begin with a venture like that. So we take an easier road -- but that road is not the way we were meant to travel.
In our reasoning about the things of God, if the first premise is not Jesus, the last conclusion will not be fully Christian. Jesus is our foundation, and we build from there.
I am not against systematic theology. But if we assume that Christ is the truth, then the best theology would begin and end with Christ; the best theology would center around Christ. The best "systematic theology" might very well be a biography. In the Bible, God has given us the right kind of book. Our systematic theologies are like a child's notebook, where we copy down pieces we do not yet fully understand. The more fully we understand, the more closely our systematic theologies resemble the Bible.