Don't disagree about Suarez.
IMO if Shaw struggled after the All-Star break the Cubs would have made a serious effort to add Suarez to be the starting 3B and Hoyer would have added Castro to be a utility guy. But Shaw got hot and Hoyer decided to ride his top prospect. And according to plan, Hoyer added his utility player.
Brujan & Berti started a total of 11 games during June & July. That means, not many games off for Hoerner, Swanson, Shaw, Happ, PCA, Tucker and Suzuki. Less than 1 game a month. So that's Castro's role, to prevent the core guys from wearing down. Not being a slugger.
But since you brought it up, Castro hit 10 HR's with the Twins this year. Last year he had 48 extra base hits, so he is a solid extra-base hits guy. Similar power to Swanson & Busch. Castro is the ideal utility guy- switch hitter, solid power and can play 2B, 3B and all OF spots.