Here is a crazy idea, but one I have always wondered about. I have not done much umpiring in my life which I consider a wise decision. However I have spent countless hours behind the backstop watching pitchers.
I really got the feeling I could see pitches better from behind the backstop than right behind the catcher. I know I have seen balls and strikes that were called the opposite way. In fact I would bet on it. The wider vision of things just makes it easier to see, especially when it comes to both sides of the plate.
Even if this were true, the other HP umpire responsibilities would make it impossible. I'm sure good umpires would disagree with my theory. But for me, seeing a larger picture makes it easier to see strikes and balls. At worse, I believe you can be at least as accurate as standing behind the catcher. Once again, I understand I might be all wet and that removing the HP umpire simply can't be done.