Jake Arrieta posted ERAs of 4.66, 5.05, 6.20, and 7.23 in his final season before the Cubs traded for him. His strikeout rate, walk rate, home run rate, and even his groundball rate all sucked prior to his acquisition. He was 27 at the time.
Obviously, I am not attempting to equate the two. Jake Arrieta was an all-time dumpster-diving success story. I don't expect nearly the same from Pearson.
The point is that sometimes you see something in a player that you think you can fix. And then sometimes you fix it. Presumably, the Cubs think they've got what it takes to fix Pearson, and with the obvious physical tools that he has that could make him something special. I wont begrudge the Cubs for giving up two dumpy prospects for a guy who could obviously be pretty darn good if things break right for us.