A catcher [individually] brings himself to the park: his work ethic, his talents, his attention span, his powers of observation, his rapport with pitchers, and his arm. That personna gets imprinted on each game situation, subliminally to the catcher - almost instinctive - and in the moment.
It is the sum of what he brings to the table that we evaluate; and our perspective [in the sense of these threads] is post-mortem, considering information, standards, ratios and standard deviations that the catcher pays no mind as he plays the game.
I would incorporate pick-offs by catchers into my metric, if I had a uniform data base for reference that dated back to 1901. It would make sense to me that it would be a considerable modifier to extra bases taken by runners on base when a batted ball is put in play. The threat of being picked off, keeping an on-base runner from both taking any generous lead AND getting a jump start from that lead would be worth - what ... a second, a second-and-a-half ... two seconds? I'd bet it's in the two second range; but I can't prove it.