Joel Garver offers great apologetic for lectionary preaching:
If one were to use the lectionary over a twelve year period, focusing the homilies during each three year cycle upon a different one of the readings, it would be possible in that time to preach through all four Gospels, almost all the NT Epistles, each Psalm, and about 60% of the OT. Moreover, since the readings are generally related in some fashion, it would also be possible to show the inter-connections between various parts of Scripture.
