You don't need to "guess" how much to pay someone. The market dictates that. HR has pay bands. Raises are always in a narrow band. I don't know what kind of disputes you're talking about.
If you ask any manager - describe your responsibilities succinctly, they will most inevitably fail to do so.
I can imagine a company that had pay bands so narrow and bonuses so small that managers had no tools to reward high performers, and although I agree that it would take away a lot of what allows managers to do their jobs, I don't think that is the usual situation.
