Another issue: To pull off using system font stacks, our web page will look different in MacOS vs. Windows vs. Linux vs. Android vs. CromeOS vs. etc. This makes testing a lot more time consuming. With webfonts, as long as we use good fonts which work with the lowest common denominator (75dpi Windows displays using Chrome, in my experience), we don’t need to add a new testing platform each time a new OS starts to gain traction.