It's nice to theme, but not as cost-effective as generic characters.
As for Black Guard, I know DivineEdge has sung their praises, and I'm not going to flat-out say he's wrong or anything, but don't take one view as gospel. Perhaps it differs from area to area, but in my experience Black Guard have been fairly mediocre, due to their relatively small (compared to the rest of Fantasy these days) unit size. Executioners with the ASF banner have been better for me personally. Again, I'm not saying DivineEdge is wrong, but just want you to appreciate that one specific unit is not going to win you a game by itself, and that there is a whole spectrum of opinions out there; it's best to find what works for you.
Give a man a match, and he's warm for a day.
Set a man on fire, and he's warm for the rest of his life!