just because a player was a power hitter as a player, it does not mean that he will expect all of the other hitters to be power hitters, or that is the only style of hitting that he can cater to. See Skip Shumaker, who McGwire has worked with.
For example, Rod Carew was a contact hitter, and a renown bunter. He was supposedly so good at bunting, it was said that you could put a handkerchief on the ground, and he could bunt the ball where it would land on the handkerchief every time, no matter where you put it.
Anyway, he was the hitting coach for the Angels and the Brewers, and like McGwire a HOF caliber player. Some of his hitting disciples- Jim Edmonds, Tim Salmon, and Garrett Anderson- all power hitters.
To me, at the MLB level, the main job of the hitting coach is to basically try to detect flaws that the batters go through during the course of the season and how teams are attacking them at the plate- and they all will have slumps during the course of the season- and basically sit down with video and try to help the hitters figure out how teams are pitching them, how to help them adjust. And when I say flaws, I'm talking about very minor things like keeping their shoulder in, and not let it fly open, and stuff like that. Not totally retooling their swings or anything like that.