your confused with resolution I think
I've had a 15" retina since they came out.
I also make use of QuickRes app to get access to actual pixel resolutions rather than the dumbed down 'best more retina, more space, etc' options you get in OSX.
The most real estate option give you 1920x1200 effective resolution (i.e. your internal rendered 4k resolution which is then displayed on the panel).
It is the resolution I usually run the panel at and provides a nice sharp picture (which takes some getting your head around since you are running an LCD at a non-native resolution which was always a no no until now).
but you are not getting access to the native 2880 x 1800.
Using QuickRes you can set that. This results in a true 2880x1800 desktop (i.e. looks similar to my imac27).
This does make everything tiny, but for some coding across multiple windows, FCPX while using multiple 'tracks', some photoshop work with lots of windows it is awesome.
Another reason is that the 1920x1200 resolution doesn't work as a user would expect with 'retina aware' apps. I expected this to be fixes by now tbh, but it never has been....
for example If I have a true 2880x1800 photo, and I am running 'best for retina' mode (i.e. my desktop real estate, etc all looks like 1400x900), when I open a retina aware app like photoshop, and view my picture '1 to 1' it will take up the screen exactly (not twice the height of screen). i.e. the OS knows that certain frames should not be doubled and antialiased but have the contents rendered 121 for pixels on the screen.
If I now use QuickRes to change to 2880x1800 resolution, and do the same thing, I get the same results as far as viewing the picture is concerned (it takes the full screen), but now all the OSX menus, etc are all rendered 121 too - so I can get much more of that stuff on the screen (i.e it acts as a regular screen before retina doubling stuff started muddying the waters)
If however, I choose the furthest right OSX 'more space' resolution (which as described above is effectively 1920x1200 as far as OSX furniture, menus, text, buttons, etc is concerned, internally it 'thinks' it is displaying at 3840x2400 (with 4 times as large OSX furniture, menus, texts, buttons, etc)
So the result when I again open iphoto and view my 2800x1800 picture is that it only takes up 2/3 of the screen in 'view 1 to 1' option. You have to in fact zoom in to 133% before you get a true 1 2 1.
this is the same with all retina aware apps. e.g. FCPx, PS CS6, etc. Its a bit of a pain to be honest and I'm amazed it has never been fixed. I mean the OS knows you are in 1920. It knows this is going to be rendered at 4k, so it knows that when displayed at native any retina aware screen windows need to be be scaled to 133% to be true... and yet we are now a full OSX version on and they have done FA to fit it.