Right and you cannot set the resolution to scaled 1920X1200 then. Oh, wait, you can.
You could also download the setres command line tool to set it to the native resolution any time you wanted.
I've successfully run my rMBP in 2880X1800 mode.
Guess what Sherlock, you're right but for the 'best for retina' scenario all that is worthless - kind of 1/3 of what makes these displays really interesting is stuck with less screen real estate than my 2 years old MBA - there is NO sound reason to keep the 13" display stuck there - your smart reply doesn't change that.
And why did you mention your 15" again?