More screen real estate seems pretty pointless if it requires you to have a pair of reading glasses on you at all times. I'd much rather prefer it on it's "normal" setting then, where everything is the same size as on a "normal" screen, except crisper where applicable.
Clearly it doesn't to some people, since Apple already sells higher resolution displays in the MacBook Air 11-inch, MacBook Air 13-inch, MacBook Pro 17-inch, and MacBook Pro 15-inch with Hi-Res Glossy Widescreen Display or Hi-Res Antiglare Widescreen Display options. In fact that's the majority of the laptops they already sell. So clearly it doesn't to
most people.
Of course UI elements wouldn't be half (or a quarter) the size—that'd be ridiculous.
Additionally, in time developers would tend to make use of the additional flexibility afforded by the choice ("smaller" or "bigger"). For example, a spreadsheet with 55 on-screen rows could have the cell borders be one Retina-pixel wide, and the text be exactly the same size only crisper, so you'd "save 110 vertical pixels" and have ~15% more screen real estate with no loss in readability and the text actually being
more readable.
Also, as parish says, clearer, crisper text, images, and UI elements allow (somewhat) smaller sizes to be exactly as readable as bigger, more pixellated elements. For example you could have things be only 10% smaller for the exact same readability, affording you an additional gain in screen real estate. Compounded with the above, and supposing the same effect horizontally, that'd be a ~50% gain in screen real estate with
better readability in this case.