The thing is why should we care about an arbitrary grade from a 3rd party when the professionals don't? Coaches grade their players because they know what to grade them on and other teams use tape and scouting reports to prepare for their weekly opponents. So can PFF grades really be a tool if the people who the tool matters to is more accurate? PFF grades are for media and fans only..lol. The problem gets to the last point; people using PFF grades as gospel.
That said, I will never put down the objective data that PFF provides. Their analytics are pretty top notch from what I've heard.