I did a story about Brian Dinkelman that explained how he might make the major league roster. That is an extreme longshot, but I think its helpful to think through the question of how player's specific abilities may push them ahead of better overall players. In Dinkelman's case that is his ability to play both the infield and outfield.
So the question here is why might the Twins include all three of the players competing for the center field position on their opening day roster. The starting point for this is that Hicks has to win the center field competition. If he doesn't, it is very unlikely Terry Ryan will let Gardenhire keep him on the major league bench. He needs to play every day.
Mastroianni is virtually guaranteed the fourth outfield spot if he doesn't win the starting center field position. He can play all three outfield positions, he is a great base runner and he hits enough to play every day for a stretch if needed. He is probably better suited to be the 4th outfielder than he is to start in center field.
Keeping Benson as a fifth outfielder is a lot more dicey. The real problem for Benson offensively is that he strikes out a lot and his batting average is not all that great. His strengths are his power and he has drawn a fair number of walks in the minor leagues, so his on-base-percentage has been better than his batting average might indicate. However, Benson is also still only 25 years old, so he might be better off playing every day in the minor leagues and refining his hitting skills.
There are reports that Gardenhire wants some "pop" on the bench. If Benson shows he will make enough contact to use his power, he would give Gardy that. He also can play all three outfield positions with plus range and arm. And a late inning defense with Mastroianni and Benson replacing Willingham and Parmelee would give the Twins three center fielders in their outfield.
Its an unlikely choice. But its not an impossible one. Even with a third catcher, the Twins still have room for an extra infielder or extra outfielder. If they decide to carry five outfielders its possible Hicks, Mastroianni and Benson will all be around for opening day.