Happ has actually been passably decent in center field. It's Kyle Schwarber who really hacks it up out there in left field. The only frustrating thing about Happ is that his bat is preventing the defensively superior Albert Almora (he would have caught it) from playing regularly.
At the end of the day, it's not just about defense or offense. Both matter. If the Cubs believe they'll score/prevent more runs with Happ and Schwarber than Almora and either slugger, then it's the right play. I only comment because the defensive woes have been so noticeable.
Elsewhere, Bartolo Colon does all of the best things.