It's just another way to group pitchers in terms of level of talent. I don't see the issue. let's keep it simple. ace means one, so your ace is you number one starter. there are thirty teams, and in the theoretical universe of "this guy's an ace, this guy's a three" or whatever, each team will get one of each pitcher, so we need thirty aces. let's even keep it so simple that we just use era to decide who the aces are. whoever was in the top thirty of era among starting pitchers in the major leagues is an ace. carlos zambrano's era of 3.77 was good for number 30 in the majors. ACE MATERIAL The whole identifying pitchers by imaginary rotation slot thing is stupid, but when people do it, they usually have an implied "on a good team." The 30th best pitcher may be a No. 1 somewhere, but he's not a No. 1 on most good teams.