ZolaIII :
I am actually glad they accomplished it as this will finally end ridiculous PPI race on smartphones as even those with best eye sight among us can't see even this much. To be honest with you 300 PPI is considered as a good photography quality with is exactly what 5"(4.8") 720p is delivering.
MeteorsRaining :
A couple years ago people were doubting 4k's usefulness as a television screen. Now we are seeing 4k smartphones coming up.
It's not the screen resolution or PPI which matters. It's the angular resolution. 20/20 vision is the ability to distinguish a line pair with one arc-minute of separation (1/60th of a degree). That is, both the PPI and viewing distance matter. For a printout viewed at arm's length (approx 24 inches), that's 1 / (24 inches * tan ( 0.5 * 1/60 degree )) = 286 PPI, which is where that 300 PPI figure for "good photographic quality" comes from. Likewise, 4k on a 50" TV is useless when viewed from further than about 8 feet. But on a 30" monitor viewed from 3 feet it's a marked improvement over 1080p.
If you hold the display closer than 24 inches away, then your eyes are capable of resolving higher than 300 PPI. So yes, those 400 and 500 PPI phone disiplays do provide some benefit since often people hold them closer.
800 PPI is starting to get ridiculous though (unless you've got a magnifier put in front of it). The only use I can see for that is to compensate for (lack of) subpixel rendering on a RGB stripe. Pentile RGBG is symmetric vertically and horizontally. So the same subpixel rendering algorithm works in both landscape and portrait mode, and you don't need to get to ridiculous PPI to fully fool the eye.
RGB stripe is asymmetric. When you do subpixel rendering on it, the effective horizontal PPI ends up being 3x greater than the vertical PPI (for monitors). That is, a 300 PPI display with subpixel rendering has 900 addressable PPI horizontally, but only 300 addressable PPI vertically. For a handheld device that's flipped between landscape and portrait mode, this corresponds to the display being different (markedly worse in resolution) in the horizontal or vertical direction depending on the orientation of the display. One way to counteract this would be to move up to something like 800 PPI - vast overkill in one axis in order to compensate for a shortcoming in the other axis.
Or you could just use the simpler and cheaper solution - switch to a subpixel array which is symmetric in both directions, like Pentile RGBG.