Across the three-year cycle of the Roman lectionary, there are a total of 49 occasions where at least one short form of a reading is optional. One blogger has suggested that this translates into between 20-28% of Masses on Sundays and Solemnities in any given liturgical year having shorter forms offered.
Most of these shortened readings, both for the Lutheran form of the 3 year lectionary and for Rome's, involve simply removing a few verses from before or after the main section of the reading. In the Roman lectionary, however, there appear to be some edits in which internal verses are, in effect, edited out.
I fear that the sole reason for using the shortened version of the readings is to shorten the length of the service. In even the most extreme cases, this is done for the sake of a couple of minutes. But at what cost? I understand the constraints of time. We have two Divine Services every Sunday with a Sunday school and Bible study hour in between. I well appreciate the need to keep to a schedule. At the same time, I fear that choice of the readings is based solely on the desire to shorten the service and this option is chosen simply because the option is provided. This is, in my view, the worst place to edit the liturgy.
If time is really a constraint, then there are a number of choices to shorten the Divine Service and to do it with sensitivity to the liturgical integrity of the rite. Sing fewer hymns or choose hymns with fewer stanzas (I abhor shortening hymns as much as shortening the readings). Preach a few minutes less. Look at the possibilities for a more efficient (pardon the term) method of distribution. If a choir sings, have them sing an integral part of the liturgy and not an added anthem. Oh, and radical thought -- start on time. In any of these choices, the liturgical integrity of the rite as well as the readings from the lectionary are treated with the respect they are due and not as things to be omitted for the cause of time.
I think this should not be a radical thought, although I fear it is. We look at the Divine Service as if it were somewhat a burden or chore that we ought to dispense with as quickly as possible. Truth be told, I seldom have a Divine Service on Sundays that is shorter than 70-80 minutes. In part this is to remind those assembled that God's time is not ours, that we do not clock watch before the Lord nor do we judge God on the basis of brevity. What is happening is too great a mystery and miracle to be counted with a stopwatch. At the same time, there is no need to prolong the liturgy without purpose. We have had services of several hours (an ordination and installation within the Divine Service) and we have weekday spoken liturgy that routinely lasts 45 minutes. Time is a consideration but not a primary one.