They're all there.

I can't believe I never noticed it before, but all three of the most hated groups in Jesus' day (sinners, tax collectors and prostitutes) are represented in Luke 15, and not in hard-to-discern, allegorical ways either. Look:

• Sinners: they were the ones who rejected the Father, asking for something that isn't there's to ask for (Lk 14.11-12). Torah, and thus vindication on the last day, belonged to God's covenant people Israel. If you read the verses carefully, you'll see that it's not about money at all really; it's about love and family. The rejection of that necessarily includes the lost of the inheritance.
• Tax Collectors: the issue with tax collectors was not really about money either. They were gathering Israel's resources (including people and land occasionally), sometimes dishonestly, and sending them off into the hands of pagan oppressors (Rome specifically). This was the betrayal that they were hated for: they squandered the inheritance God promised to Israel in foreign lands (Lk 15.13).
• Prostitutes: they come up twice in the narrative. The Greek word used for "dissolute living" (asotoce) carries the implication of sexual immorality, if somewhat vaguely. It's more explicit in 15.13b. The Greek word used there is pornwn, which can only mean one thing given the context. The idea Luke wants us to get is this (in 20th century American imagery): John-Boy asked Pa for his share of the farm, which he then sold in order to move to L.A. so he could live like a rock star and blow it all on heroin and porn stars.

Of course, all of this makes perfect sense given the fact that Jesus is accused of identifying with (and building his movement around) marginalized, excluded people more in Luke than anywhere else.

Why would that be?

Because Luke was writing to an audience who felt excluded from the gospel: Romans and other Gentiles primarily, but by extension all the others who didn't make the cut for whatever reason, inside or outside of Judaism. Luke 15 says the same thing that Matthew 5.25-34 says. If a sheep or a coin are worth so much to the one who has lost them, how much more valuable is a lost person to the Father who loves him/her?

All have value. None are excluded now that Jesus is Lord.