One of the most beneficial and effective forms of preaching, in my opinion, is expository preaching.
1. The whole council of God is eventually taught with expository preaching.
Where as pastors who preach topically tend to preach on there favorite themes week after week. I was once an attendee of a friday night service at a local church in boston. For the 8 weeks I went, at least 6 of them eventually turned to a message against sexual sin. The problem was that these teachings were being taught out of sections of scripture that it had no correlation to sexual sin what so ever. What we were getting was the pastors favorite topics, not the text.
2. Expository preaching forces the preacher to teach each passage in its proper context.
Whether it be an argumentative flow of thought, literary genre, historical context or otherwise. No verse can be taught contrary to its context. Thus the preacher is more likely to cone to an accurate understanding of the text at hand.
3. Expository preaching ensures that the preacher preaches on he "hard"teachings of scripture.
If a pastor has committed himself to expository preaching, he will likely preach through particular books of the Bible. This will force him to touch on all areas and themes that scripture presents. Whether it be the doctrine of Election or the Syrophoenician woman, all themes will eventually get dealt with.
If your pastor doesn't preach in such a fashion, I believe he is doing a disservice to his people. For it is the fruits of expository preaching that brings about spiritual maturity and growth within a congregation.