I think this parable is about faith. I don't think that prayers are more 'effective' for repetition. A simple inarticulate groan is 'heard' just as much as repeating a prayer over and over. I think the point of repeated prayer is to stay 'in the Spirit' and so in a state of receptiveness to what unfolds. Prayer imbues us with the gifts of the Spirit and we are transformed. But to answer your question, I don't know. Perhaps the parable was to encourage the 'practice' of prayer the way a parent teaches a child something that they should do, or not do, even if they can't fully understand the reason for it. I do not think that 'God changes His mind,' even though there are stories in the Bible where this seems to be the case (the sparing of Isaac on the mountain and Abraham's intercession for the people of Sodom come to mind) all prayers have been heard and answered before the world was born, it is our perspective that changes through prayer. It is about our faith, which the parable of the unjust judge and the one I picked out about the lilies both to me seem to address. Here's another that comes to mind:
Once again Jesus seems to be harsh, if not unjust, and then with pleading he changes his mind. I think it points to the way the Gospel spread to the gentiles after first being presented to the Jews, but it is also about steadfast faith. Interesting though to think who exactly was transformed by faith in this one.
just ramblings,
luna