A human eye with 20/20 vision can distinguish ~150ppi at 2 feet (a typical screen-to-eyeball distance). Beyond that, it's overkill. The apple thunderbolt display is at 109ppi, so there's a bit of room for improvement, but not 60% improvement.
Beyond this limit, I suppose there are people with 20/1 vision or something, but they're pretty rare.
Beyond this limit, I suppose there are people with 20/1 vision or something, but they're pretty rare.