This affects mainly Windows users (why I do not know.. but the default Google fonts have always look like shit on my Windows machine).

If you don't see any difference think about how many people are viewing your app/site on windows machines and having to look at the horrible default rendering.

You can always tell when a site is build with Apple and not throughout tested on Windows, the fonts are always horrible, and there are very few tested services that render fonts across OS/browsers the right way, so this is a great idea.