Yeah, at first he seems like the standard, wise old mentor figure whose main purpose is to serendipitously arrive and guide the main characters. They're definitely aware of the archetype because they subvert it for comic effect later on. But he has his own history, motivations, and agenda.
They practically explain what his character really is: an older version of McNulty, who's wised up enough to make peace with the system. Drawing parallels between a black and a white character in this way isn't a huge social statement, but it's also something that rarely happens on TV, like Jay Bilas comparing a European prospect to a black NBA player.
PS If you like the Wire, you should think about reading David Simon's Homicide book. It's really good, crisp narrative nonfiction and you'll be able to spot anecdotes, lines and ideas that were used in the show. Harry Edgerton was obviously the starting point for McNulty and Freamon, just like he was for Andre Braugher's character in Homicide.
PPS Don't believe your mom, border. People were schtupping like rabbits according to Kinsey. If there was less premarital sex, it's cause there was a pretty sharp drop in the marriage age post-WW2 (which everyone then internalized as being "normal").
