A good pickup pitcher is not going to make the other players on the team better, but defensively the number of strikeouts should go up, the number of walks should go down, the WHIP should go down, batters should see less favorable counts, and fewer balls should be hit off the "sweet spot" of the bat. All of these things add up to a much better chance of the team being successful. There are a lot of A-level teams who would quickly become B-level if they lost their ACE pitcher, and there are a lot of B-level teams who could compete at A-level if they added an ACE.