Interesting statistic. I think it works in giving an overall picture, but like with any statistic, it can sometimes be misleading. Take the 2007 Giants for example. I know they had some early season injuries on the defense, but come playoff time that defense was playing at an elite level yet statistically they are below 16th in points allowed. Making it appear that Eli won a SB with a sub par defense.
The 2016 Cowboys. That was not a good defense. They were hidden by a potent ball control offense. That D was every bit as bad as the 2014 team who BTW also came in at better than 16th for the exact same reason.