There are some churches which have merely expository sermons. In those, the goal of the sermon is to convey the meaning of a given Bible text, being not unlike an academic lecture. Therefore the quality of the sermon is exactly in line with the quality of the exegesis.
But for any Christians who have any further purpose to their sermons, exegesis serves to further a rhetorical point (and these can be many). Therefore the quality of the sermon is determined not only by biblical exposition, but by the preachers argument.
As someone who has specialized in the study of the Bible, I can speak to the exegesis portion readily. As for sermon rhetoric, I am mostly an amateur critic. Allow for me to present the four basic situations I encounter when criticizing the rhetorical use of biblical exegesis in sermons:
- A valid exegetical observation is used to make a good rhetorical point.
- A valid exegetical observation is used to make a bad rhetorical point.
- An invalid exegetical observation is used to make a good rhetorical point.
- An invalid exegetical observation is used to make a bad rhetorical point.
Numbers 1 and 4 do not warrant comment. Numbers 2 and 3 are the tricky situations. I confess I am much less likely to be critical in the case of #3. For example I'll drag out the textbook "root" fallacy:
The Greek word for "church" is devired from roots which mean "called out," so the church is being called out from the world.
It is an absolutely silly exegetical observation, but I support the overall point - that the church is called out to be distinct from the world. I generally accept those sorts of statements, because the overall point which came across is good. If I were to be critical, it would be fairly low-key.
Number 2 presents an uncomfortable situation. I am having trouble thinking of an example, because it seems to me that if 1) the exegetical point is valid, and 2) the argument is sound, I ought to support the conclusion. Yet I am nonetheless more likely to be critical of a preacher in such a situation, because it seems that they are trying to use the truth of scripture for bad purposes.
The implication of this observation is that I think that the rhetorical message is more important than the exegetical observations in a sermon. I think that is a good thing, overall. I am dubious of preachers who claim their sermons are merely expository, but even if they were, why would you hear such a sermon during worship? It seems to me that sermons need to have some sort of devotional or encouraging or hortatory component in order for them to fit in during church services. At the same time, I worry about sermons which are purely rhetorical and do not rest on proper biblical exegesis.