Thats true of everyone, as you say. So you have to include the good with the bad when you look at these guys. For example, Kuroda has had three bad starts out of nine. But in five others he has surrendered 2 runs three time, no runs once, and one run once.
They ALL count in figuring out who is performing.
Beckett and Lester are middle of the pack statistically; they are not aces, not right now.
Verlander, Peavy, Lowe, Morrow, and Hellickson are the guys pitching like aces right now. We don't have an ace.