Intentionally? Makes sense. Not like they manufacture dedicated external displays with built in MagSafe adapters.
Let's assume best faith. siglesias might have just mean "that design choice makes no sense", not "you make no sense".
On to speculation - I'd guess they don't want you to use full screen, and only included it as a concession to things like movies / games.
The first time I used the feature on a Macbook Air without an external display I suddenly 'got' it. It made a lot more sense.
(That said, at least Mavericks doesn't make it unusable on a multi-display setup.)
Edit: Incorrectly thought fullscreen mode was introduced in Lion
It was.
On the contrary, the Lion fullscreen mode, which caused the issue, was a relatively large push towards fullscreen, by giving it a standardized and better (if you don't have multiple displays :) UI and integrating it into built-in applications.