Two reasons for Diaz:
Taxi Squad players still workout and take batting practice with the team, the only thing they don’t do is play. They still get the rest of that experience where they hit, field, ect with MLB coaches right now.
The other reason is why Hyde had McKenna on the last trip or two: Get him used to the daily MLB schedule, with travel, hotel, meetings, workouts, ect so that he has hit feet wet when he first gets “The Call.”