Using golf as an analogy, I liken Oracle's practicing against their teammates to a golf pro competing against his or her caddy. While Emirates would be more like the golf pro out playing on tour against other pros, where one shot can mean being eliminated from the tournament. Oracle was in the Cup regardless. The challenger has to earn their way and when something like that is on the line, you work harder to get there.
On the surface, I think your point makes sense. But these guys are professionals, and there is serious coin being invested in this. They have the data and a room full of analysts looking at every aspect of every second of ENTZ's prior races. They have rooms full of engineers looking at every aspect of the boat and its performance. And the sailors were great sailors before this competition.
And they know exactly who they are practicing against, and the limitations of such. There is no excuse for them suddenly getting an extra six or more knots out of their boats a week too late.