For the record, Priester looked worse than the likes of Kyle Harrison and Dustin May when we traded for him.
Teams don't just trade for a stat line on a piece paper, or a 3 month track record nor do they trade for a what a guy did while playing away. They're trading for what they think they can turn a guy into. I don't see many people complaining about Lucas Giolito, who was a signing not a trade but still they thought they could get him right.
Eliminate a pitch, add a cutter, add some velo, mix up the pitch mix, change an arm angle. Teams look at guys differently than us. They also must see the forest for the trees, which us as fans often do not.