What the Bible itself teaches often differs considerably from the ways one uses the Bible to teach.*
How true it is that the Bible, for many, is a prop for a pet point. Preachers often run to texts because they seem to support a notion that they have already decided they want to preach. This is a dangerous mishandling of the Word of God.
I was reminded of this a while back when I was studying Romans 14. Many people run to Romans 14 in order to defend certain positions that they hold on "debatable practices."
While Romans 14 may give principles about how to make decisions, that is not really the point of the text. The point of Romans 14 is about how living sacrifices (cf. 12:1) relate to people in the body of Christ who view God-honoring things differently than they do.
So in preaching Romans 14, we must preach primarily about relationships, not about decision making. Decision-making is a key biblical concern, and Romans 14 certainly helps us in our thinking on that topic, but it is secondary in Romans 14.
As a preacher, to preach is to stand up and proclaim what God has said. Therefore, we must be careful to say what God has actually said. We must make the point of the message the same as the point of the text. This is the most basic definition of expository preaching.
Along the way, we may deal with implications, sub points, side points, assumptions built in to the main point, and like material. But we should be careful not to preach a side point as a main point.
Where there might be an exception is when a text is used for an extended series. In subsequent messages, a side point may be the main point of that particular message, but even in these cases, it must be conscientiously and constantly tied back into the main point.
*David C. Deuel, “Expository Preaching from Old Testament Narrative,” in Rediscovering Expository Preaching: Balancing the Science and Art of Biblical Exposition (by John F. MacArthur Jr. and The Master’s Seminary Faculty; edited by Richard L. Mayhue; Dallas: Word, 1992), p. 285.