John Drury offers an interesting and very different assessment. Drury looks at the story of the corrupt judge from the perspective that the long narrative section between Luke 9:51 and 18:14 ("one of the great riddles of gospel study") is a Christian Deuteronomy -- "a handbook on the Christian life in the historical setting of s journey to Jerusalem, just as Deuteronomy is a guide for the devout Jew set in the historical perspective of the journey into the promised land with Jerusalem, the place where God will cause his name to dwell, as its centre."
Next Deuteronomy insists upon the judge's duty to condemn the guilty and to acquit the innocent, setting Luke to his more sophisticated parable about the godless judge who did justice out of self-interest. We have seen him mixing the blacks and whites of Deuteronomy into grey before. The parable is an obvious pair with the friend at midnight (Lk 11:5-13), having the same construction -- the grudging giving of men as a shadow of God's generosity.(Tradition and Design in Luke's Gospel, 140)
There is something very attractive about Drury's understanding of Luke as a self-conscious historical narrative designed to instruct the reader in the dynamics of faithful living. Whether or not we accept his historical assessment of this saying, that seems like an interpretation of Luke-Acts worth retaining.
The Jesus Seminar identified the core parable (vv. 2-5) as possibly coming from Jesus:
It exhibits the kind of unconventional features that are characteristic of the parables Jesus told: the judge grants the widow’s request not because her case has merit or because he is impartial and just in his judgments. He decides in her favor to be rid of her. He wants to avoid being harassed, perhaps to avoid having his honor or reputation beaten black-and-blue (such is the implication of the Greek term used here) by her continual coming to demand vindication. (Five Gospels, 368)
The Seminar understood v. 1 to be an introduction created by Luke to provide a framework for the parable, while vv. 6-8 reflect other Lucan interests of prayer and parousia.
Bill Loader notes the "playfully shocking" character of this story:
The parable is not unlike that story Jesus told about the rogue in 16:1-8. Here, too, we may be dealing with a story that was going the rounds at the time and which Jesus picked up and used. The woman was likely to be such a nuisance that the judge relented and dealt with her case. Good on her! In 18:6 Jesus turns the attention of the listener to the unjust judge and proceeds in 18:7 to make a statement about God. If this kind of judge was willing to respond to this poor widow, can’t you believe that God will respond to us? The point is not that God is also corrupt, but that God is likely to respond. The parable is playfully shocking in the way it is prepared to liken God to the judge. This enhances its effect.
In a similar vein, Gerd Lüdemann affirms the historicity of the core story in vss 2-5:
These verses, the original stratum of the parable, certainly go back to Jesus. That is supported by (a) the criterion of growth, for here with vv. 2-5, calculated backwards, we are at the third stage, and (b) the offensive character of what is narrated. Normally a judge should pronounce judgment on a legal basis. But here he is godless and pronounces judgment only because he is forced to. (Jesus, 375)
John P. Meier
Most interpreters put considerable weight on the fact that Luke himself understood this parable as a model of prayerful perseverance. John P. Meier notes this intention by Luke when discussing the relationship between this parable and the short eschatological discourse that precedes it in Luke 17:
With regard to the eschatological discourse in 17:20-37, some may wish to include the parable of the widow and the judge in 18:1-8 as the concluding section of the discourse (so Feuillet, "La double venue," 5, 25-28). Notice how the reference to the Son of Man in 18:8 forms an inclusion with the mention of him at the beginning of the main section of the discourse in 17:22 + 24. However, the separate introduction at 18:1, indicating that Luke understands the parable mainly as an exhortation to persevering prayer, weakens the possible connection of the parable with the preceding discourse. (A Marginal Jew, II: page 477, note 107)
Schweizer suggests a historical process as the tradition gave different meanings to the story at various stages:
The earliest form of the parable culminated in "how much more." It inculcated the certainty that God hears the prayers of the oppressed "widow," i.e., the community, which prays for the coming of the kingdom. The parable would be entirely altered if the appellant were a wealthy property owner with all kinds of connections, who could bring a variety of pressures to bear in order to receive vindication. The parable thus also tells the community that its prayers reflect its relationship with God. It cannot, need not, coerce. It can take comfort in the knowledge that it is entirely dependent on God and learn to pray even in times when prayer seems totally meaningless. God in freedom will give the community the kingdom, when and wherever the time comes. This confidence springs from the fact that it is Jesus who tells the parable (or that the community traces it back to him), for in Christ the kingdom is already coming to those who can really hear the parable.
Later it became important that the kingdom come soon (vss. 7b, 8b). The admonitory question in vs. 8b was also added. Finally, Luke takes a sober look at the community of his own day and recognizes that prayer plays too small a part in its life; he, therefore, puts more emphasis on the summons to pray (vs. 1). In the context of 17:22-37, he is probably thinking of prayer for the coming of the Son of Man. (Good News according to Luke, 280)