Is it possible to only run one application as non-retina whereas everything else is retina

Images and icons look about the same because of integral scaling. But text in non-Retina apps looks clearly worse. The difference is that on a non-Retina display, text uses sub-pixel rendering. But with Retina, old apps use full-pixel anti-aliasing instead. (Those full pixels are then multiplied perfectly with integral scaling.) Not quite sure what Retina text on a Retina display uses, it's hard to eyeball.

Separately, other resolutions look pretty good. The highest is 1920x1200. The trick is they are also rendered in double-resolution (3840x2400) and then scaled down for the display (2880x1800). Because there are so many tiny pixels, the result is acceptable, but the optimal setting is definitely sharper.

So Retina apps at 1920 look pretty good, and non-Retina text at 1920 has smaller and slightly fuzzier full-pixel anti-aliasing.

It's likely that the external display connections are simply wired directly to the discrete GPU. The internal display is connected to a gmux chip that handles switching between GPUs and it probably would have been significantly more complex to route the 3 external ports the same way.

I think it's fair to assume that most users connecting external displays will have AC power available, so this seems like a fine way to simplify things.

It's definitely not an issue of graphical power - I use an external 2560x1440 monitor with the new Iris Pro-only 15" rMBP and previously used it with an Intel HD 3000-equipped 13" MBP, both of which handle it fine.