oh.. you're comparing 1080p at 4.7in vs 5in

Huh? That's a difference between 294ppi and 469ppi.

Stereoscopic vision is effective at a distance of [distance between pupils / diameter of pupils] times that of focus.
3D on movie screens is effective, even when not simulating something super close.

3D is only one of the benefits of the device, anyway. The wide field of view and head tracking are what makes it special.

With the low resolution of the Ocular Rift, and small aperature of the human eye, I doubt focus will be an issue unless you are simulating something within arm's reach. Even then, I doubt it will be a huge issue if you don't spend to much time looking at things up close.

It can later be fixed by gaze tracking, refocusing the hardware lens on the device and rendering the focus blur in software.

Steam DRM doesn't restrict my hardware choices, so it's better than iTunes music (pre-DRM-free) or video, where I have to use an Apple device or Windows to play the file.

The iPod previous generation iPod Touch is labled as having a Retina display, yet it has poor color gamut and viewing angles (it isn't IPS)
The iPhone 4 had a fairly low gamut display. (And I think the 4s was the same)

I love high pixel density, but I think better viewing angles are even more needed. I'd like to see IPS become the standard like it has for LCD smartphones and tablets.

The HD 3000 is at 4%. All Intel GPU options combined is 10%.
There is just less fragmentation in the Intel market.

The problem is that often the "technical specifications" are meaningless or misleading, like a TV marketed like this:
HD, 600hz, 170 degree viewing angles, 2 ms response time, 1,000,000:1 contrast ratio and over 2 billion colors.

Retina has a useful meaning to me: It means that the PPI has exactly doubled from all the previous products in the line*, while keeping all the GUI elements the same physical size. So it's also a software thing.
No, it's not useful when comparing with other devices, except that Apps made for Apple devices are more likely to target the exact resolution of the screen.

In the case of the MacBook Pro Retina, the longtime standard for notebooks and desktops was doubled (They tend to be close to 100dpi)

Hence the need for a term.

The screen on the Surface is about equivalent to the one on the $399 iPad, though. Of course, all of the specs are different, the Surface may be better in some ways

In some ways, it's worse on OS X. The dock never shows any icons at their native pixel resolution, so they are always less crisp than the same icon in the windows 7 task bar.

The reason for multiple sizes is so elements can be perfectly aligned to the pixel grid for maximum crispness. A vector image won't help in this case. You can lose up to half the effective resolution by not designing for an exact pixel size. Of course, this becomes less important at higher PPI.

There is an x86 Surface that runs full Windows and has a digitizer for pen input. I have a New iPad, but the (ideal) x86 Surface is closer to what I want.