Between my computer engineering background, and the view I have of things without my glasses, I’m hyper aware that what I see on the computer screen is an illusion. Each of those curvy letters is really an array of blocks stacked up, red, green, and blue blocks are even separate, and you only think you’re seeing a white pixel. Ideally the distance between blocks is smaller than the smallest discernible difference the eye can see, but when you look at the “O”, and you think you have no blur, that’s just your brain filling in the missing bits of information to make that blocky display and speckled rods and ganglion compression into a smooth curve.
It’s less of an issue with print, but at some level it also is rendered in dots per inch, not smooth curves. Text starts at 300 dots per inch, can be much finer with laserjet. Ink runs slightly though, so the dots in a field tend to merge together and produce smoother edges. Cheap printing on color packaging though is sometimes still obviously dot matrix on the images.
Not saying it isn’t a useful measure. You do want to know when the amount of blur is more than your visual cortex can compensate for. Degrees of blur are real, but what we perceive as clarity isn’t necessarily directly related to the focus of the light falling on our retina.