I have found the role of the older brother in this story quite fascinating as well. Of the three parables told to the same groups at the same time this one alone addresses the issue at hand, in that he speaks directly to the pharisees and scribes about their attitude toward the "sinners" Jesus is investing time in.
I have to believe that this is significant in the understanding of the story. The proud, selfish desires of those who remain in the faith can overshadow the true and significant finding of the lost. I dont know how many times I have encountered people who say they desire to see the lost saved, but then when the lost come the house of worship the very same people who cried out for their souls wish the lost would have chosen another place to seek out God. We are all lost without Christ, for crying out loud!!!
Many times it does not makes sense to us how in our faithfulness we find the same grace as the one who has wondered off and then returned. It seems only fair to us that they should recieve a lesser gift or a smaller portion of grace. We only need to look as far as the closest mirror to find the one has "earned" less grace. That is the wonder of the gospel.
His grace is indeed sufficient for each one of us. Those who remain 'faithful' and those who do not.