The problem for Revere is that we aren't really talking about a fourth outfielder, its really a fifth outfielder behind Young, Span, Cuddyer and Kubel. While Kubel can DH most of the time, getting Thome in the lineup requires one of those four guys to sit occasionally. The fifth outfielder is really only needed to give Span a break in center field and as a late inning defensive replacement. Repko or Revere, as fifth outfielde,r would play sparingly while also being used as a pinch runner.
Of course you could sit one of the other outfielders to give Revere at bats. But, with Nishioka back to add speed to the top of the lineup, its hard to see how Revere is more valuable to this team than a power bat in the middle of the lineup Of course, that assumes the Twins get everyone healthy and Young, Cuddyer and Kubel are providing those power bats. But given the rest of the lineup is producing, it is going to be tough to find regular at bats for Revere. While he's an exciting player, I think his overall development is going to be better served playing every day at Rochester than watching from the major league bench. Come August, if the Twins are 10 games out of first, that would change. But not while they are still trying to get back in the race.