For whatever is worth, this approach is similar to how it is handled on the Linux desktop-- for KDE at least-- apart from third party applications, icons are handles as as separate package.

EDIT: I wanted to reply to W00t: latest comment...

And by the way, this allows for easier theming anyway. Custom icon packages have to include icons for the common applications as well... and the desktop standard should handle fallback to a default "hicolor" theme if there is no custom icon in the theme.