The idea of anti-aliasing is to make edges look smooth so it is nicer for human eye to see. But lately with the availability of high-resolution monitor, with high density per inch, which some called as "retina display", is anti-aliasing still required to improve the quality of the image? With such high ppi, human eye is no longer able to distinguish adjacent pixels, so it may look unnecessary to implement AA that will only burden the GPU. Am I understanding correctly?
I'll keep this brief. Yes, anti-aliasing is still very relevant. The reason is that the human eye can see subpixel movements on most displays. You'll often see pixels per degree as the metric used and reference 60 PPD for "retina" displays. The human eye at the center of vision has hyperacuity of small pixel movements that go past 400 PPD. So things like rendered lines that are moving on a high contrast background can appear jarring without AA even though they are very tiny.
Try this demo: http://jsfiddle.net/w5se983j/4/ Walk away from your monitor and you'll see the pixels moving.
That said these AA techniques usually only apply to animation and thin lines. It's very possible that if all you're displaying are images with very little contrast between adjacent pixels that AA will offer very little.