Oh! Yeah, that makes sense. I think they used McNulty as an example because that quotation is his/used on him and because he's the...catalyst? to get the overarching plot going (I don't think they're following through on how McNulty ends up when they apply his words to other characters/stories; I think it's more that they are using 'giving a fuck when it's not your turn to give a fuck' as a starting point for character motivation & plot -- like, at the start of The Wire, I think McNulty gets involved for no other reason than because he's lonely and bored and wanders into the right room at the right time (the somewhat anti-hero-ness of that appeals to me. Heh).

And I like that with this trope you don't even have to like the character that much! (because I don't really like McNulty, but I don't mind watching him because the show isn't about him as a main character just because he's the white dude, and I don't get the impression that I'm supposed to like him or think he's right all the time either.)

(Come to think of it, H:LOTS does this with Tim Bayliss too, though I think perhaps he's not quite as much of an wretch as McNulty.)