Actually I've found threshold of zero to be perfect. If you blitz seven you might get burned, just like in real life. With the threshold at zero it makes me play
actual defense. With the default settings (50 threshold) you can blitz seven and if you get burned your D-lineman with 78 speed will catch up to and tackle Chris Johnson and his 99 speed while he's breaking away down the sideline... Not real IMO.
Also, with the threshold at zero,
player ratings are actually taken into account. When I played against Peyton Hillis he would truck through some of my defensive backs. On default threshold my nickel corner could tackle him by himself using one arm.
Passing becomes more accurate as well. Linebackers won't super leap across the field to make ridiculous interceptions or pass deflections. Your and your opponents players become true to their ratings.