Thanks to discussion on Spenser's Sneakers, I've finally hit on why I object to Spenser's calling killers for backup. Not that it reflects badly on Spenser—he has never asked anyone to kill because he couldn't do it himself—but it lets Parker skirt serious moral dilemmas.
In MORTAL STAKES (1975), which directly precedes Hawk's debut in PROMISED LAND, Parker has Spenser make the tough decision to commit premeditated murder. It's possible Parker thought, "I can't have him go that dark again. Let me bring back Hawk to help with the terrorists in THE JUDAS GOAT."
Spenser remains committed to good. "Good", to him, isn't necessarily the cops or the courts, but more a streetwise sense of what's right. He respects Hawk and various ersatz Hawks because he sees some good in them. They all keep their word. Their use of violence is well controlled. If they're killing bad guys to protect the main good guy, they're on the side of the angels. If they ever killed randomly while working for Spenser, as they say on Firefly, that would be an interesting day. But Spenser calls them precisely because they don't kill indiscriminately. He couldn't continually call cops for backup; they might not have the free time or the jurisdiction to kill if it were necessary.
Parker evidently became so comfortable with the trope of deadly friends watching a hero's back, he carried it over to his Sunny Randall female P.I. series, turning Spike—a smartass waiter played by his son Dan in the Lifetime Spenser movies—into Sunny's karate master friend.