I'm not trying to be an ass or pedantic, but I know what retina is. What I quoted above tells me you do not. Retina is just marketing vocabulary for "high pixel density", but that's another discussion entirely.
If things followed you logic, the second one zooms in on an iPhone(which also has a retina screen), the retina screen suddenly stops being retina. I think we both agree this is not the case.
The screen is always at native resolution, it's the graphics card that is rendering it at different resolutions. If you use the "best for retina" mode, that simply means everything is pixel doubled. The screen is at native resolution, but the effective resolution is half that(1440X900). Changing the screen resolution via software is does not change the physical number of pixels, thus the screen is always retina. Even if you set it to 800X600, it's still retina, as in, you still cannot distinguish the individual PHYSICAL pixels in the screen.
So long as you keep the same aspect ratio as the screen (720p is 16:9, your screen is 16:10, which is why you said it didn't look that "grate" (sic)), the image should look just fine.
If you were to run a 15's retina screen at ACTUAL native resolution, all icons and text would be laughably small, and most people would be unable to read them.
Which is exactly why I said that (if it were actually possible), gaming at native resolution on a retina MBP would render every GUI item extremely tiny. It's safe to assume it would render most games unplayable.
Thus, I think OP should not look at a retina screen the way he does.
Before I get started, I don't want this to become an argument. Maybe I didn't word something well before so I'm going to try and re-word it now. I am very familiar with how retina works, not only from personal use but as I worked for AppleCare for 5 years.
I honestly didn't say anything about it not using all the pixels. Its always going to use all the pixels but at lower resolutions it uses more pixels to produce what a single pixels does. Example, a pixel in a 1920x1080 image on a 1080p screen only uses one pixel, but that same image on a 4k screen now uses 4 pixles to display the same result that took only one pixel before.
Zooming in on an iPhone is not the same things as changing resolutions its still retina just zoomed in, its still at the same pixel density.
What I was saying, and this was right from Steve Jobs (may he rest in peace), is that Retina is Apples term for "At normal viewing distance for this size screen, the resolution (pixel density) will be high enough so that your eye can not tell the difference between individual pixels offering a perfectly clear image." At any distance its still going to be a Retina screen or a screen capable of displaying "Retina". However, if you follow the formula for what Retina actually is, its only truly a retina image at a certain distance and only at that native displayed resolution.
Also, saying that if the screen was at Native text and application icons would be small is no longer a true statement. For years Apple has been making high resolution icons so the icons are only as big as their rendered size or as small as you want to shrink them to. In the system preferences Apples display settings can be misleading, as you have Best for this display which for the 15in mimics how we are used to things looking on a 1440x900 display at that size while the scaled options are just that, scaled versions that make the screen appear as how your accustom to seeing it at those resolutions, but its all native retina.
Detailed Description of Retina:
When introducing the iPhone 4, Steve Jobs said the number of pixels needed for a Retina Display is about 300 PPI for a device held 10 to 12 inches from the eye. One way of expressing this as a unit is pixels-per-degree (PPD) which takes into account both the screen resolution and the distance from which the device is viewed. Based on Jobs' predicted number of 300, the threshold for a Retina Display starts at the PPD value of 57 PPD. 57 PPD means that a tall skinny triangle with a height equal to the viewing distance and a top angle of one degree will have a base on the device's screen that covers 57 pixels. Any display's viewing quality (from phone displays to huge projectors) can be described with this size-independent universal parameter. Note that the PPD parameter is not an intrinsic parameter of the display itself, unlike absolute pixel resolution (e.g. 1920×1080 pixels) or relative pixel density (e.g. 401 PPI), but is dependent on the distance between the display and the eye of the person (or lens of the device) viewing the display; moving the eye closer to the display reduces the PPD, and moving away from it increases the PPD in proportion to the distance.
It can be calculated by the formula "2dr tan(0.5°)" where "d" is the distance to the screen and "r" is the resolution of the screen in pixels per unit length.
In practice, thus far Apple has converted a device's display to Retina by doubling the number of pixels in each direction, quadrupling the total resolution. This increase creates a sharper interface at the same physical dimensions. The sole exception to this has been the iPhone 6 Plus, which renders its display at triple the number of pixels in each direction, before down-sampling to a 1080p resolution.
