I can't believe almost every major sports site has started citing PFF recently. Worst stats site of all time.
The problem is that sports fans (or more specifically, NFL fans) as a whole have this immense desire for games to be compartmentalized through statistics, to be able to logically predict outcomes and also to look back and a significant stat that is of such immense consequence that it makes the result clear cut and understandable and predictable.
I think this stems from how statistics can be applied to other sports, to make results more foreseeable. The problem is that individual stats don't translate nearly as well to the game of football as they do with other sports. Football is the ultimate team sport; other sports more closely resemble a series of individual matchups.
Over the years we have seen a sizable increase in the number of individual statistical categories in the game of baseball, and sports fans who follow baseball want (and expect) the same abundance of stats for the game of football. All those stats translate well in MLB because it's primarily a one-on-one game (pitcher vs batter), but the NFL is like an orchestra, with each player's individual performance so heavily dependent on the performance of others.
Fans don't recognize that failing and demand more individual stats; sites like PFF are simply fulfilling the most basic of economic clichés about supply and demand. the fact that the numbers are of dubious merit is secondary to fulfilling the public's desire for this type of information.
It's not all that dissimilar to how often we see fans apply baseball logic to football roster building (e.g.,
trade for Larry Fitzgerald!) that ignores the vast difference between contracts and the salary cap in the other sports, to the unique nature of the cap and contracts in the NFL.
Most football fans also follow other sports, and as a result some of that 'other sport' mentality bleeds over to the way that they view football. Because of that a site like PFF flourishes - despite their erroneous and inaccurate information.
While not perfect, a far better source of statistical information would be Football Outsiders. They realize the futility of individual statistical analysis of certain positions (e.g., center, guard and tackle) but instead compile information on entire units (offensive line). It's too bad that the members of the media that cite PFF do not instead rely on FO for the statistics that their followers enjoy reading and hearing.