Why does Bill James include unearned runs in his Game Score (GmSc) formula? On the whole, how useful/effective is this metric in evaluating pitching (can either be James', or the revamped FanGraphs version)?

Jon Lester's last start -- which included 3 earned runs over 7 innings -- resulted in a 41 (James'). Pretty interesting, I think.

And I've heard there's a GmSc formula for hitting -- anyone familiar with it?