A study was done breaking down the differences between MLB umpires from 2007-10. The difference in runs scored between the pitcher and hitter-friendliest umpire was more than two per game (7.91 vs. 10.16).
And some umps even incorrectly called pitches more than 30% of the time.
My own opinion is to have the homeplate ump wear a small earpiece and let GameDay (or a similar system) call balls and strikes. If it's a strike, the ump hears a beep and motions for a strike. A ball means no beep. For the time being, we'd still need an ump there for plays at the plate, to give balls to the pitcher, to watch for balks and catcher interference, and to help on fair/foul calls.
Inevitable poll -