Earlier quoted context omitted.
You're unhappy with the readability of San Francisco? I've heard the opposite from most people who have seen it on the Watch. I think it's quite a beautiful typeface, and I'm glad they're expanding its usage to their new keyboards and iOS/OS X. I think it will work out great for them.
It does look good on the Watch. But that doesn't automatically mean it would be a good choice for other uses. In my subjective opinion it looks pretty bad as OS X system font. Take a look: http://9to5mac.com/2014/11/19/how-to-use-the-apple-watch-fon...
To me, it looks really great as a system font.
The reason is that it is legible. That legibility comes thru the junk on my glasses and the fact that my prescription is out of date so things are just slightly blurry. Yes, I need new glasses.
But that's a proxy for people who don't see as well. Apple's serving everybody. And not all of us can see so good.
There's an argument that this line of thinking will take Apple down the path of mediocrity-- lowest common denominator and all that. But hopefully it's only an option and they continue to use the "Best" font (of which I'm not the best judge) for the system.
And then make San Francisco an option as a system font in accessibility.