As much as defensive categories can help make a fantasy league more "complete", negative stats (like errors, or walks allowed) can be an incentive to not play a full slate of games if a race is tight and if a league doesn't have minimum games played/innings pitched. In a standard 5x5, you already have three relative categories, which in my mind is more than enough. I hate it when someone tries to protect his ERA/WHIP, for instance, by not pitching all his innings. Just like a real-life manager could'nt not send a pitcher to the mound because he only has poor options, a fantasy owner shouldn't be able to sit on his butt and "unplay". MHO.