This seems like a compromise. Someone knew that the standard text was somewhat hard to read, so he tried to change the standard setting, but someone with more power wanted to keep it. So they implemented the larger size as an option.
It's not larger, it's bolder.
They could have named the option "Make text bolder" instead.
What a bizarre setting. If that's "enhance text legibility", then the "off" position is "make text harder to read, please"?
"Off" means "prettier".
Prettier? "Off" means "Make my font look as ugly as Parse's logo".
You realize it's the same font slightly bolder, right? Are you sure you scaled the image 100% to compare them, and your browser didn't shrink the image down to fit in your window or something?
Good design gives you a peak of usability and aesthetics. If you need a switch to let you prefer one over the other, then I think you've done it wrong.
Usability and aesthetics are both subjective. For some portion of the population, you will always be "doing it wrong."
You just made the most important comment in this entire thread. This should be the top post. Apple: your move.
I agree, the default type is too thin to use for body text. I have a real problem with the gmail app doing just this.
If I built iOS 7, I would have created these settings specifically for the dev beta and instrumented them. Knowing that 58% of dev users turn on the extra legibility setting is good info for how heavy to make the text in the production release.