Evidently, mainline seminaries are pushing the use of the lectionary, and many liturgical resources websites tailor their contents to the lectionary. It has become almost ubiquitous in the mainline denominations. That means that in a pastoral career of 30 years, a preacher will "work through" the lectionary 10 times, basically going over plowed ground again and again. There are two things that I personally really, really don't like about the lectionary: first, as I've already suggested, it leaves out much of the Bible including many important passages; secondly, it focuses the preacher's attention on the text in the context of the lectionary rather than the context of the congregation. Preaching should always be in, to, and for the congregation hearing the sermon. There are times when a church needs to hear a particular message, and sometimes the lectionary passages for that particular week might prove useful to that end. Other weeks, the preacher has to ignore the needs of the church, or mangle scripture to make it fit those needs, or ignore the passage. All of these are bad choices. They are lazy choices. The lectionary, if used slavishly as evidently many do, can create a barrier between the preacher and parish.
It can also relieve the pastor of the necessity of wrestling with the meaning of the Bible for the church. The Monday morning question is not what parts of scripture are relevant but rather what are the lectionary readings for this week? Now, it is true that one rationale for the lectionary is that it keeps preachers from just going back to their favorite passages and narrow parts of the Bible week after week. It "forces" preachers to preach from many parts of the Bible. That is a problem, but this particular solution is no better than the disease itself. In fact, it is a palliative approach. It also limits the preacher to what parts of the Bible are used for preaching, and because it always includes passages from the gospels, the rest of the New Testament, and the Old Testament, preachers can focus on what type of material they like to preach from—the popular choices are the gospels or the letters of Paul.
At the end of the day, however, the key point is not whether or not to use the lectionary. The point is, rather, that preachers preach a message relevant to the current life and needs of the congregation based on relevant passages. It is not "wrong" to use the lectionary frequently or even regularly so long as the option to ignore it for a week or a year is always on the table—and so long as the preacher is ready to use non-lectionary passages that are equally as valuable and pertinent as those contained in the lectionary. Used circumspectly, the lectionary can be an aide to worship rather than a crutch or like blinders. That's the point.