It's important to point out that just because it's at 0, doesn't mean there will be lockdown coverage on every single play. Overall, however, the CPU does a better job of covering the pass with the slider at 0. I found that at a higher number, I could complete passes not only easier in a specific play, but more often. I can't do that at 0.
And the reason I have it at 0 for my defense is simply to make the game more "even". I want my defense to step up and play smart occasionally. To control the ability of the CPU to pass, I give them higher accuracy. This seems to make the CPU more aggressive throwing the ball, while the 0 reaction slider gives my guys the best chance of occasionally making a play.
I agree that the slider may be "broken" in that there is not a glaring difference between 0 and 100. It's more subtle, but I do think it is there over a large sample size of passes (and games).