Solarte can play SS. He can't do it well but he can do it. Did it nearly 30 times last year. If you factor in hitting, defense, baserunning, and whatever else makes up the value of a player, Solarte is the better player at SS. The best player should play, not the best defensive player.
I bet you if Solarte played a full season at SS and Gift did as well that Solarte would end up with a positive WAR and Gift would have a negative one. Solarte is probably at least a win better actually.
But it's whatever. It's not a decision that will make or break the game I just wish they'd field the best team they could.