Exactly. The same reason position players have stamina, control, pitching splits and a P1 (fastball) rating. Doesn't mean they're going to be used, but you can't block them out just for position players.
I guess all non-catchers could conceivably have PC set to zero, but just like a non-pitcher may have to pitch in an emergency, a non-catcher may have to catch in case of an emergency, so you'd want the sim to select the best option available, therefore there should be SOME rating available off of which to base that decision.
And if the question then becomes, why not zero it out for pitchers only? My answer would be, why single out pitchers? If my only remaining catcher gets hurt late in an extra inning game, and I have one bench player left whose AS/AA/PC ratings are 56/44/12, and a long reliever in my bullpen whose ratings are 77/83/34, I'm going to want the sim to put the pitcher behind the plate to finish the game.
(There's a rule in real life that says, in this exact same scenario, that the game's original catcher can be returned to the game if healthy, but I doubt this rule has been programmed into HBD.)