The fact that people don't like the idea because of defensive metrics makes me love the thought even more. The important numbers you didn't reference for Castro?
.215/.336/.398/.734 and .235/.312/.393/.705
Your Luddite commentary about defensive metrics aside, Wieters is posting a .760 OPS this year compared .734 for Castro, and has a career .744 career OPS, so how the heck is he a big improvement offensively exactly? Beyond that he's constantly injured. When you combine that with his horrible defense he would be a horrible signing.
The nice thing with Castro is his line against righties: .250/.377/.485 .862
He would be a perfect platoon guy with Flowers.
Wieters just sucks all around.
And if you think having a good defensive catcher isn't important when our pitching staff is going to be insanely young then I don't know what to tell you. I mean, take a look at the difference between our pitcher ERAs with Flowers versus AJ, it's not even close. It's not a matter of metrics, it's just common sense.