Per the new update:

"The manager will no longer bring in a defensive replacement or player for rest at catcher when it's in the 7th or later and the starting pitcher has a no-hitter going."

-Is catcher ERA a real usable statistic or is this just a cosmetic thing that is true to a real game? And if catcher ERA is important to the sim, then why...

1.) would a better defensive catcher not be better to bring into the game?
2.) does there not seem to be sortable statistic for this in the draft center (or maybe I'm missing it)?

As far as I know, it's worthless. I'm not going to go through the performance history and compare it to real life anytime soon.

I've never considered it of any use. Your point #2 seems reasonable to me

C ERA means absolutely nothing in the sim. Nothing. Doesn't affect anything. The only factors affecting how good a catcher is defensively are his defensive ratings; range of course matters for almost nothing unless you play the guy out of position. I think the reason for this change is to try to better simulate reality - in real life a manager would never take a pitcher's catcher out of a game during a no-hitter unless the catcher became injured. It's the same thing as the update years ago that set it such that a pitcher will almost certainly not be pulled during a no-hitter, even if your settings indicate that he should be. Even if these things don't make sense in the sim, a lot of people want that little dose of reality in the game and admin obliged.