With more screens having higher and higher image resolution, some of the sprite don't always look so clear as they once did. And the font awesome code is virtually pure css, for easy and fast loading.
Every mobile device I have tested, including the old Nokia style phones (early 2000's), can indeed process fonts and the css code. So you've got a wide range of compatibility, including IE7 support (even though you're only supporting IE8 I believe).
This would improve the image quality regardless of the screen size or screen resolution, while improving load time, and maintain device compatibility.