Even at resolutions where humans can't resolve synthetic patterns of single pixel lines a single pixel apart, they can often see stair-stepping in lines and curves of text. While subpixel rendering and smoothing does help, there are limits to what they can do.
As technology matures, why settle for things that barely outresolves most people most of the time? In most cases 12 bit 24 KHz audio would be enough, but today we use 24 bit 48KHz (or more) audio because it outresolves everybody all the time.
Just as we currently may poke a bit of fun at the "640k ought to be enough" that Bill didn't say, I wouldn't be surprised if we one day will poke a bit of fun at the "300 ppi ought to be enough" that Steve didn't say.