I'm actually not sure that PC actually does anything at all. I wouldn't be surprised if they just put it in there and then never got around to making it functional.
In Regulars World, my C has a 47 PC. Here are his catcher ERA's over the past 4 seasons:
710 Inn 3.86 ERA
1022 Inn 3.27 ERA
1038 Inn 4.03 ERA
710 Inn 3.88 ERA
I've used various high PC guys as backups, and they have all been in the 84-88 range. Here are how they have performed:
577 Inn 4.41 ERA
441 Inn 3.74 ERA
441 Inn 4.08 ERA
756 Inn 4.17 ERA
That's definitely a relevant sample size, and the guy who's 40 points worse finished with the better catcher ERA 4 years in a row. How can a rating be relevant if bad outperforms elite 4 (!) seasons in a row?