Before I reveal his answer, consider two definitions. Topical sermons are just like the name suggests, you choose a topic, find a few scripture verses dealing with that subject and build a sermon. Expository sermons expose the meaning of a passage of scripture. To preach an expository sermon, a minister must first learn what the passage meant to its original audience, then he must relate it to the lives of his listeners. To preach a topical sermon only requires a concordance.
Topical sermons can be good sermons.
Like all roads which led to Rome, all of his texts led to water baptism. The next Sunday, the good reverend was back in the pulpit and by way of introduction, he repeated the meeting with his deacons. The danger of such preaching is that it leads to imbalance. A minister's favorite subject can be expounded to the neglect of all other topics. His listeners will become malnourished, starved for good spiritual food, or malformed, denied essential spiritual vitamins.