You say that as if they are in the same category, Tyler Wells is a fringe #5 starter. Is he better than someone they kept instead? Maybe, but pitchers tend to be crap shoots anyway. I'm losing zero sleep over him. (My first thought when I saw his name was "Who was he again?")
The fact that Castro was only 26 meant there was still some time to get better. Through age 25 he was a below average hitter, but not way below. If he could get to average with his athletic ability and versatility, he would be pretty valuable. He exceeded that. It's not a Shane Mack type of coup, but it was a good find.