I am currently reading books from the Humble Bundle ebooks collection. Just plain regular text books on my tablet is fine, but the comic ones (SMBC, XKCD) are not readable. It is nothing to do with the size, and everything to do with the tablet screen being 150 ppi. Being able to read that is not "utterly useless".
As you can see from many of the comments, people do want to buy higher pixel density screens on their laptops. Apple even came out with laptops where that is the distinguishing feature. But if your choices are outside of Apple then simply cannot buy high pixel density screens.
Laser printers when they first came out were 300 dpi. That is a very good indication that those kind of pixel densities make for better legibility. Sure you can read stuff at 75dpi, but it isn't as productive.
GUIs go to great lengths to work around the low ppi, using anti-aliasing, (auto) hinting, subpixel rendering and various other techniques. This kind of thing generally isn't available for bitmaps such as the comics in the Humble eBook Bundle which are consequently extremely difficult to read (the images are a higher dpi than the screen so it is a screen problem not a source problem).
You are right that there are people for whom extra ppi, extra cpu, extra memory, extra power saving, better colour gamuts etc won't make a difference. But there are also a group who do want ppi improvements and Apple has done so across their laptop and tablet lines, Asus has done for some of its tablets, and the Nexus 10 has done so too. If nobody bought those then it would demonstrate lack of demand, but people have been buying them. And people really want them for non-Apple laptops too as Linus stated and others have concurred.
In that case your argument should be that "retina" ppi should be the target. 150ppi as on my tablet definitely isn't. 130ppi as on my laptop is not either. I join Linus and others calling for higher ppi displays on laptops. There is debate on the exact number but it would certainly be at least ~220 ppi on laptops and ~300 ppi on phones/tablets.
As you can see from many of the comments, people do want to buy higher pixel density screens on their laptops. Apple even came out with laptops where that is the distinguishing feature. But if your choices are outside of Apple then simply cannot buy high pixel density screens.
Laser printers when they first came out were 300 dpi. That is a very good indication that those kind of pixel densities make for better legibility. Sure you can read stuff at 75dpi, but it isn't as productive.