The bottom line is that I work with text (source code) all day long and I would rather read from a display with laser printer quality than one where I can see the pixels like an old dot matrix printer. Some displays are getting close to 300 DPI which is like a laser printer from 35 years ago.
The brief version is that if someone has a screen real estate concern, they need to look for the PPI, but if they have a visual quality concern, they need to look for the PPD.
Maybe it will be elucidating if I describe a scenario where you will have low PPI but high PPD at the same time.
Consider a 48" 4K TV (where 4K is really just UHD, so 3840x2160). Such a display will have 91.79 PPI of pixel density, which is below even standard PPI (that being 96 PPI, as mentioned).
Despite this, the visual quality will be generally excellent: at the fairly typical and widely recommended 40° degree horizontal field of view, you're looking at 3840 / 40 = 96 PPD, well in excess of the original Retina standard (60 PPD), which is really just the 20/20 visual acuity measure. Hope this is insightful.
It also introduces an element of uncertainty: as you say, you can't specify a laptop screen's PPD since that's dependent on viewing distance. But that's exactly the problem: it's dependent on viewing distance. Some people hunch over and look at their laptops up close and personal, others have it on a stand at a reasonable height and distance. To use PPI is to intentionally mask over this uncertainty, and start using ballpark measures people may or may not agree with without knowing.
To put it in context, for this display, "Retina resolution" (60 PPD), i.e. the 20/20 visual acuity threshold, is passed when viewed from 47.09 cm (18.54 inches, so basically a feet and a half). I don't know about you, but I think this is a very reasonable distance to view your laptop from, even if it's just 12.2" in diagonal. It corresponds to a horizontal field of view of 32°.
> the 20/20 visual acuity threshold
The acuity threshold for random blobs of light.
The threshold for sharp edges is much finer, and the things we put on computer displays have a lot of sharp edges. https://en.wikipedia.org/wiki/Hyperacuity
Won't deny, since again, PPD depends on your field of view.
Yes, if you shop for "resolution and diagonal size", you may as well shop for PPI directly. This just doesn't generalize to displays overall (see my other comment with a TV example), as it's not actually the right variable. Wrong method, "right" result.
> The threshold for sharp edges is much finer, and the things we put on computer displays have a lot of sharp edges.
And the cell density is even finer. It was merely an example using a known reference value that lots of people would find excellent; I didn't mean to argue that it's the be-all end-all of vision. It's just 20/20.
It's wrong but it's wrong in a way that causes minimal trouble and there's no better option. And if you add viewing distance explicitly, PPI+distance isn't meaningfully worse than PPD+distance, and people will understand PPI+distance better.
This might sound like a nitpick but I really don't mean it to be. These are proper well defined concepts and terms, so let's use them.