PFF brings objectivity and rationality to such conversations.
PFF is so far removed from "objective and rational" that it's hard to respond to your post if that's the starting point, but a couple quick responses: The McDonalds comparison is because it's a bad product that is massively consumed way more than it should due to convenience and marketing. It's not "trusted by media" -- it's used by media because they were either paid to start using it, were ordered by their bosses to use it (some of those bosses being self-interested investors in PFF, see Cris Collinsworth), or because the media is lazy and its numbers are easy fast food to point to (only when PFF agrees of course) to make them "look smart". And I think we have a different idea of "knowledgeable fans"... I'm not talking about the blowhards that think they know everything, I mean the people that are knowledgeable enough to know that they don't know.
In a nutshell that's the whole problem with PFF, it pretends it knows something it doesn't. PFF numbers are in reality no more trustworthy than those "knowledgeable fans" you dislike, in many ways much worse than a knowledgeable fan, and in fact PFF is much, much more harmful overall because of this false "air of legitimacy" given to it by its believers.
Watching Eric Reid yesterday was a good reminder of the PFF problem. I don't have the exact PFF numbers, so I may be slightly off, but three years ago PFF graded the rookie Reid among the best safeties in the NFL, and the media rejoiced at how he therefore deserved to make the Pro Bowl, and all Niner fans jerked off in a circle because PFF proved their rookie safety was the best.
Meanwhile, in Indy there was a bad-to-average safety (according to PFF coverage numbers) named Antoine Bethea. He was signed by the 49ers as a free agent. Magically, PFF instantly graded Bethea among the best safeties in the NFL. And once again the media rejoiced at how Bethea deserved to make the Pro Bowl. And Niner fans again jerked off in a circle, of course.
But poor Eric Reid. While Bethea was getting this overnight love, all of the sudden those PFF grades that loved Reid just a year before now suddenly showed he wasn't anything special, or the more diplomatic "took a step backward". He's now an average-to-bad safety according to the magic PFF numbers. So, should we go back and take away Reid's rookie year Pro Bowl because obviously his true talent is now showing? Do we blindly just agree with PFF that Bethea's talent magically switched from bad-to-jeebus because he put on a Whiner jersey?
Or perhaps we should think that the change in personnel, or perhaps injury, affected Reid's individual performance? And perhaps the change in team & scheme affected Bethea's individual performance? Obvious, yet the PFF method cannot account for the most important part of football -- team. PFF can never, ever do what people want to use it for -- objectively measuring individual talent with scientific accuracy.
The problem is casual fans use it exactly for that purpose ("x has a 4.3 grade, y has a 4.2, so I'm right x is better than y and you're wrong!") while the more knowledgeable fans will acknowledge it for nothing more than it is -- a convenient, subjective way to see what players are having a good year under their unique circumstances. Which is something that knowledgeable fans already know without giving money to PFF's snake oil salesmen.