I don't know what tangible effect a hitting coach has on a team. In my limited experience covering baseball, I've found a good hitting coach is someone who can help bust players out of slumps by getting them to slow down and better understand their swings. Maybe he'll help them make an adjustment. It's not unlike therapy.
I don't believe a hitting coach can make someone better, but a good one can at least help a few guys squeeze a little more out of whatever talent they have. Whatever. It's not like this team is a hitting coach away from winning the division.
That said, I'm glad the Pirates have brought in Jay Bell as the new hitting coach. He was my favorite player growing up; the progenitor to Jack Wilson, the capable and sure-armed shortstop whose sidearm action to first I tried to imitate, much to the chagrin of my little league coach. He was a completely average ballplayer in nearly every respect -- the kind of guy the Pirates would have killed to have on the team last year.