I hadn't really thought about this aspect, but ESPN's article noted:
Adding the robot umps is likely to cut down on ejections. MLB said 61.5% of ejections among players, managers and coaches last year were related to balls and strikes, as were 60.3% this season through Sunday. The figures include ejections for derogatory comments, throwing equipment while protesting calls and inappropriate conduct.
That is a benefit for umpires - people can't complain about calls when they can get them overruled.