Klyne Snodgrass writes, "The parable is one more example of Jesus substituting the mercy code for the holiness code" (Stories with Intent, 358). I agree. Jesus always seems to have an agenda which doesn't quite match up with our assumptions of what is important. An interesting piece of background to this story is not so much the ins and outs of what priests and Levites were expected not to do around a corpse, but rather the exceptions to the law that should have come into play here. Since the victim in the story is half-dead the purity laws were put on hold for the sake of a life (cf. Snodgrass, 355).
Here are examples of a priest and a Levite coming from Jerusalem - the seat of holiness and the blessing of God upon the world - who believe their personal purity is worth more than even the exception to the law that they could have optioned. It is the Samaritan, who is quite unwelcome within the cultic and racial divisions of Israel that places the work of life above self. Therein lies a great lesson for us within this story: we are called to show God's love, not maintain our own righteousness.
Yes, the mercy code is more important than the holiness code. I think we still often act as though God will be unable to handle our 'defilement' in the world even when we are working on behalf of his love and his kingdom. Jesus came to destroy that approach and establish a new way.
The message for the church: Show love and let God worry about your holiness.
The message for the church in culture: Worry less about image and more about kingdom.
There is more to the Good Samaritan parable, to be certain. But this is a significant small piece that should not be taken lightly . . . and never ignored.