It is very hard to argue with scholars of such great influence and ability as I. Howard Marshall, whose impact is still reverberating through the world of New Testament scholarship. Marshall is one of those scholars who had his hand in everything. He was able to work at the theological, exegetical and textual/linguistic levels. Marshall reigns supreme in his own work in the Gospels and in his commentary on Luke, where he makes an interesting comment on Luke 8:4-8, the parable of the Sower.
By comparing the redactionary relationship to all three synoptic gospels, Marshall says right at the offset:
The section 8:4–8 is based on Mk. 4:1–25. In Mk. this is part of a longer section (4:1–34) on the parabolic teaching of Jesus, which is expanded by the inclusion of further parables in the corresponding section in Mt. 13. Luke, however, has abbreviated his Marcan material at this point.
Here, Marshall gives us the proper context to his argument by laying out the data in the other synoptics. I would like to highlight the quote where he suggests that “Luke, however, has abbreviated his Marcan material at this point.”
Most scholars would say that brevity in discourse or a lack of description is an indication of being an earlier version of a story. It is assumed that because you have less data, your story will be shorter. With the Gospels, we do not know where they get their data from, so we must assume that having less information is an indication of being more archaic.
We even use this principle in textual criticism. When we have a shorter reading and a longer reading of one text, we assume that the longer reading is an embellishment. A scribe might embellish a text in order to be “clearer” on a particular doctrine or to combat a heresy that was gaining traction.
This instance of redaction is slightly different. Marshall is saying that Mark, the shorter, earlier, more archaic version is longer than Luke. Luke is redacting from Mark and intentionally utilizing brevity as a discourse tool! There are a number of reasons that Luke slices down the word count from Mark, but my main issue is the sensibility that longer is better. This has become such a staple of biblical scholarship, yet we see Luke doing something completely contrary, and admitted by I. Howard Marshall.
Marshall explains further why Luke would shorten his Marcan source material,
Then he includes the brief narrative of how Jesus’ relatives visited him (8:19–21 par. Mk. 3:31–35); by holding it over to this point, Luke is able to use it as a final comment on the parable of the sower—stressing the importance of hearing and obeying the word of God. The section as a whole is thus concerned with the theme of hearing the word of God, and the accent has been shifted somewhat from where Mark places it.
Marshall identifies this difference as stylistic; it is an issue of rhetorical strength. Taking into consideration that ancient histories were not considered with sequential chronology in the same sense that modern histories are, it would be okay to cut and paste this parable. But it still leaves the question of why he would shorten a text.
I might be splitting hairs here but I still find the question intriguing and that Mark is true to his source, and so is Luke, but one has cut a piece down to size for rhetorical value, for stylistic purposes. This makes me wonder if we could reconsider things like Marcan priority. Is it possible that the brevity we find in Mark’s gospel a stylistic one? That is not going to be answered here but it is still a good question to ask, and I fear we treat Marcan priority as a scholarly axiom. Is shorter better? Maybe so, but I think the conversation is still up in the air.
 Marshall, I. Howard. The Gospel of Luke: A Commentary on the Greek Text. New International Greek Testament Commentary. Exeter: Paternoster Press, 1978.
 Ibid 317.
 I. Howard Marshall, The Gospel of Luke: A Commentary on the Greek Text, New International Greek Testament Commentary (Exeter: Paternoster Press, 1978), 317–318.