1) As the Dead Sea Scrolls and Nag Hammadi materials are researched, the old Lectionaries and related ritual books remain the one undertapped source of early Biblical manuscripts
2) As the synagogues were using lectionaries 5 centuries before Christ, lectionaries are an important element of understanding the context of Judaism and the early church
3) Lectionaries remain the primary resource for pericope analysis
4) Lectionaries provide the context for the sermons of the early church fathers
5) How the church used scripture from the earliest records to present tells us something about the church's understanding of the meaning of the scripture
6) Nearly 80% of the Christians in the world use some form of a lectionary.
7) The framework for Lectionaries is also the framework for the Divine Office, Liturgy of the Hours, Morning/Evening Prayer. This type of prayer grew out of the Jewish household prayers and is therefore an important element of the history of the church.
Common misunderstandings:
1) A lectionary does not always dictate the readings for the day. Depending upon the denomination there are varying degrees of flexibility and choice.
2) Not all scripture specified for a day is necessarily in a lectionary in the technical sense. The number of books and the contents of each volume is variable across denominations
3) Not all entries are associated with particular calendar dates. Some entries are intended for use in times of war, natural calamity, etc.
4) Not all lectionaries cover an entire year - for example, there is a common environmental lectionary that covers 4 weeks. There are formal prayer cycles that cover one month - for example, the Anabaptist Missio Dei Brevary
5) Contemporary lectionaries do not stand on their own. It is important to be able to compare the use of a passage across time and denominations to understand how the church has traditionally used a passage
6) Calendar dates for lectionaries are of secondary not primary importance. Across the web you find two separate indices - one for the current calendar year, one by liturgical year title. The latter is the permanent identifier against which archives are kept.
7) Lectionaries are not limited to liturgical churches - note, for example, the African-American lectionary hosted at Vanderbilt.
A pretty picture to put things in perspective:

Yes, my harping about lectionaries again gives GS an opportunity to complain about interlinears and DH to gripe about Handouts. My guess is that Logos will give us 2 of the 3 - interlinears are here to stay (Sorry George)