Dash -
Just a thought - wouldn't you want a machine that throws breaking stuff as well?
I watch a lot of youth/HS ball and where I see the biggest weakness in plate umpires is inconsistent calls on breaking balls.
Of course it's real easy to be judgmental when I'm sitting in a spot where I have the perfect angle to see at what height a pitch crosses the plate... It's probably a whole lot harder to judge the height/depth of a pitch moving right at you at high speed.