You're losing physical vertical screen space, resolution is a different issue. You're getting a long narrow screen. Apple won't be making these devices any larger (goes against everything they're doing right now) so the only way you can go to 16:9 is make the screen physically smaller vertically.
It limits your working capacity when you're working in a single window, something that's far more common than working in two at once. Obviously if you mostly work in two windows you'd have a different perspective.
I've used 16:9 laptops before and I didn't care for it. Tons of wasted horizontal space and a generally very awkward hotdog-like form factor for the machine.
With the same diagonal a 16:10 rMBP has a screen which is 5.18% percent bigger, but also has 11% fewer pixels. These pixels may be wasted in certain applications, but in my experience they should often come in handy. Programs that are unable to make use of the extra workspace added in the 16:9 screen generally waste far more than 11% of the horizontal workspace. Word would be an example. A 16:10 display is only about 8% physically taller, and a 16:9 display is less than 3% wider than a 16:10 display. To me these differences seem very small compared to the other advantages currently offered in 16:9 panels. Is text easier to read because it's 8% taller? Is a 8% taller icon easier to click? I guess so, but not by very much. I don't know what kind of hotdogs you're eating that are anywhere close to the ratio of 16:9. It's actually kind of amusing to me how much people obsess over the advantages of 16:10 over 16:9 laptops, especially when you see how small the differences are in numbers and even more so when they dismiss advantages on the other side.