Well someone doesn't like advanced statistics.
sorry for brief sidetrack:
there is no statistic that tells a story without context. no matter how objectively and/or abstractly you grade a play, it does not happen in a vacuum. PFF writers continually quote their all-encompassing score for players' seasons, games, in relation to what roles they should play, what money they should get in FA, and so on. they are doing their own hard work a disservice by not trying to interpret their findings beyond "5.0 is better than -1.3" type fingerpainting. advanced models in other sports are usually cast in context by writers in much more illuminating ways than PFF does. i would love for them to realize this, because they have a lot of people's attention right now with the grading they do.