How important is this rating, and what number should I look for?

As important as any other. Higher the better.

I'm not sure getting a guy with a great and accurate arm is nearly as valuable as PC.  On the other hand, my two-headed platoon doesn't really have a great arm OR great PC.  My guy who plays against RHP has AS/AA/PC numbers of 67/63/56 but has a career batting line of .310-.371-.500 (1711 AB's/batting ratings of 81-76-59-80-77).  My vL platoon C has defensive #'s of 67/70/62.  His career batting line is .308-.366-.555 (907 AB's/ 71-89-93-54-76.  

So:  :C is nice, but a great arm might be just as nice....and don't ignore a bat in you search for glove.

But low 50's is probably my floor for PC.  

My personal opinion:  defensive catchers RULE.

I think the #s back me up on this one.  

Extreme example:   Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Dennys Franco

There's no obvious threshold for PC, or for arm quality, but 80+ always gets my attention.  Most GMs don't seem to place any meaningful premium on catcher defense, so this is a position that is often easily upgraded at little expense, and it's a very real difference-maker.  Also, because catchers tend to have lower overall ratings than other players, stud catchers are frequently still available in the 2nd round of the amateur draft.  

Hope this helps.
Zz

I use .12 ERA per 10 points in PC as a baseline.    That said, I have two teams nearing the A/S break and both of them call "Bullshit".

Aurelio Rodriguez	C	63	35	319.2	0	10	48	.208	1	3.604
Chico Reyes	C	54	38	342.0	6	19	45	.422	0	3.105

AR is 74, CR is 60.

This one is starting to normalize.  They were almost the same when this thread started.

Jaime Mitchell	C	42	37	308.0	4	16	48	.333	0	4.266
Jeffrey Hargrave	C	58	41	400.0	3	15	43	.349	0	3.938

JH is 83 to 49.

If you have a solid staff you can get away with lower pitch calling.  For me  Hardball Dynasty – Fantasy Baseball Sim Games - Player Profile: Lawrence Grimsley has awful pitch calling but my pitchers make up for it.  If you have a less than stellar staff you certainly want a higher pitch caller to make up for it and help them out.

My catcher with PC of 96's ERA Caught was a half run per game lower than my catcher with PC of 67. That's huge.

I've had these two catchers on my roster for the last 3 seasons (among other guys).  Most of the time they've been in a platoon - meaning the starter depended only on the opposing team's SP, not my SP.  Osuna was the Def replacement, so he often entered late in close games.

O'Shea (PC 42) --
S29 3.77 ERA, 777 Inn
S30 3.56 ERA, 786 Inn
S31 3.65 ERA, 362 Inn

Osuna (PC 68) --
S29 3.98 ERA, 660 Inn
S30 3.32 ERA, 678 Inn
S31 3.60 ERA, 250 Inn

That's a 26 point PC differential with little to no difference in results.

For what it's worth, my team generally has good pitching and very good defense.