and here's why- you're only going on yardage. If you give up 400 yards a game, that's 100 yrds a quarter, and since most teams probably average three offensive possessions, that's roughly 33 yards a possession.
Conversely, if you have a really good defense, but your offense turns it over 6 times inside the 20, and your defense gives up 42 points because of that, but you only hold the other team to 250 yards of offense, that doesn't mean that the defense necessarily did a bad job.
The stats can be misleading. You're twisting them to try to support your arguement.
I think they need to come out with something like a defensive effeciency rating. I think it should be based on how many points the defense allows and how long the scoring drives were. In other words, you get penalized for an 80 yard drive vs. a 20 yard drive. A defense should also get credit for turnovers created, whether the defense scored a TD on the turnover and also a way to give the defense credit for the offense scoring on a drive after they force a turnover with a TD being rewaded more than a FG.
I think that this is the best way to tell how efficient a defense is, because let's face it, the whole point is to keep the other team from scoring, not just from keeping the other teams yardage down.