I don't disagree with the usefulness of statistics in principle, because they are merely a measurement on whatever scale you wish to judge a player's contribution, and in that respect can only provide you with more information, which is never a bad thing.
But treating each stat as a discrete, singular entity is where I think you're bound to go wrong. You just can't do it with a player's contributions over 90 minutes or even over 4 or 5 games and get an accurate picture. Pienaar may, say, have 2 assists in 10 games. That stat alone, that measurement of 'final pass' productivity, suggests he hasn't contributed much. You have to augment it with, say, how many 'key passes' did he manage in the final third? How many through balls behind the defence did he accurately play? How many chances did he set up for his teammates? You start putting those stats in and you see well, he may have got only 2 assists but he played the pass before that 4 or 5 times; he managed to set up 6 or 7 shooting opportunities which teammates missed; he played 4 or 5 through balls that set someone in behind the defence. How about when he draws the fullback and winger in on him and manages to squeeze it through to Baines (somehow!) who is then completely umarked? How do you measure that contribution if Baines messes up the cross? None of our other players can create so much space, so cleverly, for their teammates. It's the kind of inventiveness that in the grand scale of statistical judgement - goals & assists - just gets lost. But watch it with your own eyes, and look a little deeper into the stats available, and you know it's a far too simplistic evaluation of his contributions.
In short: Pienaar is ace.