I actually think the main reason most superstar hitters don't coach isn't that they are bored with the game and don't want to work for you when their career is done...but more that they usually relied on superhuman talents that most hitters do not possess.
Edgar Martinez can pull Kyle Seager aside and tell him "dude...you're swinging at way too many balls...stop doing that" because...he had the batting eye of a T-101 by Cyberdyne Systems. :) But that doesn't mean Seager will ever learn to have a great batting eye.
Having SAID that...I do, in fact, believe that Edgar was more of technician and hard worker than your average superstar. Babe Ruth never successfully managed because Babe Ruth was a big dumb man's man who hit bombs, not a technician. But Edgar studied hitting mechanics...was constantly tinkering and tweaking...learning about pitchers and what they look like when they have different plans and frames of mind, etc. He also spend two hours a day staring at plastic colored balls to make his eyes not go haywire and then another three hours a day hitting and lifting and another hour a day rehabbing his constant leg problems. And then played the games. :) So...he can be in the clubhouse and see who the ones are who aren't working hard enough...he can figure out who needs to get their eyes checked. He can make tweaks to their approaches and they'll trust him enough to try it...etc.
So even though I don't think he can teach plate discipline...he might STILL be a net gain for us.