I think the only screens that have jumpped in size where pixels per inch have such an impact are the newer Mac Books with retina display screens. Something that not even iMacs have, though a 27" retina display would be a bloody monster. ...and the next iMac that I'd upgrade to! But I predict that would be for some time.

The necessity of calculating density and dpi of a monitor still remains on mobile screens and not so much desktop/laptop monitors and screens. I don't think that these values are exposed to the software level anyhow, are they?