1. Download current design file
2. Edit
3. Submit pull request
4. Repo owner downloads file
5. Repo owner figures out what has changed (somehow hopefully) and hopes nothing else was broken with the edit due to different versions, setups etc.
6. Repo owners accepts pull request (while the change cannot be seen in github since it can't diff .ai files).
Your "editing" of a graphic file is still 4 steps, there's not much being cut out:
1. Click edit. 2. Propose changes. 3. Owner decides nothing is bad, or messes up other parts of the design. 4. Owner accepts pull request (still can't see the files in Github, because there is nothing for viewing graphical diffs anywhere yet)
The great thing about hosting on github is that people can send pull request and you have real collaboration. That's alsmost impossible with Illustrator files. In reality many OSS is abondoned and change leaders or simply die. If you have a source that is accessible by 100% of the programmers (and with icons there is a huge crows who's interested) then your project probably won't die.
Want will the kickstarter accomplish? Sure it'll give you great quality icons, but then what? What if the project is delivered and put open sourced? Is it going to be maintained indefinitely? For 2 years? 5 years? What about in 10 years? Can we still use it?
But now we're really off topic, I'm just re-elaboration why open source (and line based source code) is better. I could also argue why documentation in Markdown is better than word for highly collaborative projects....