In a sense, theology IS dealing with what others have said, for they are writing what they write based on some level of authority -- typically (but not always) the Bible. Interacting with the writings of other theologians is part and parcel of the entire process.
It is HOW we interact that makes it theology instead of fighting.
There is no sense in starting over from scratch to formulate a doctrine of the church directly from Scripture as you seem to suggest. That would take ages and likely cause flawed or false doctrines -- and we tend to see exactly that when we get away from orthodox theology and venture into some new understanding. But, that being said, we SHOULD interact with fellow theologians WITH the Scriptures using sound exegesis and proper hermeneutics. Anything less is something other than theology.
To properly lay out the theology or doctrine of any particular individual, one must first read that individual, grasp what they are saying, discover their methodology (for instance do they accept allegory as a valid means of scriptural discovery?), and then decide, based on our own research in the Word whether what they wrote is valid in part, valid on the whole, or not valid at all. Perhaps their starting point is askew, and all else after that is also off the mark. In cases like that the theologian is not worth pursuing. But, on the other hand, let's say that their methods are sound, their exegesis is correct (as far as it can be, especially counting resource material available for their perusal), and that their orthodoxy is also sound, but a disagreement is found in one or two tenets of their theology. Do we then need to toss out everything they wrote based on those one or two things? I would think not!
And, isn't that the issue with Calvin and Luther, for instance? Because they both had roots in the RCC, they are looked down upon by many who have never read, nor processed their actual works. Because they continued in the practice of paedo-baptism, many discount everything they did theologically. But that really is not a fair assessment of their actual body of work. And just for fair measure, let's toss in Wesley and Arminius, two of the more famous Arminian theologians. As I recall, both also supported paedo-baptism (the Methodist church still does, as do most Arminian non-Baptist churches), and both had other errors in their theology, such as loosing one's salvation, second blessings of the Holy Spirit, etc. Yet, they, like other brothers in the Lord have done good work, written good theology, and furthered the cause of Christ! We stand on their shoulders...