So here's my 'two cents...'
Sometimes you just need a medical specialist, because not many of us are surgeons and it's hard to perform an operation on yourself.
Same goes for interpreting the Bible, sometimes you just need a theologian to help you understand it. Most of us, for instance, can't understand the original hebrew, arameic and greek, and are dependant on translations to read the Bible, translations that were made my theologians. Technically speaking you don't read the Bible itself, but what the theologians made of it, because all translation is interpretation. In principle you cannot base your exegesis on a translation, especially when you're a preacher.
Theologians are not only specialist in the field of biblical theology but also in the fields of historical, philosophical, systematic and practical theology. They're there to teach and inform others; to train people to become pastors, ministers and priests. Those become specialists aswell, because it's a huge responsibility to preach the Word and guide the 'flock.' We shouldn't depend on 'amateurs' to do that job.
Apart from that we also have to bear in mind that theology is still an academic study, that can be done for theology's sake. When my study of the Bible is purely an academic exercise, to deepen my understanding of the text itself, I do not pray the Holy Spirit for guidance first. When I want to study the Bible as an expression of my faith (so as a part of my relationship with God), I do pray the Holy Spirit to guide me first, because only through Him I can understand the text AS the Word of God.
I think someone who preaches the Word should do both: AND have a firm knowledge of everything that's related to our faith in Christ Jesus (aka 'theology') AND pray God's Spirit to guide him when he's interpreting the Bible and preaching it AS God's Word.
