I see your point, I'm not saying that GitHub isn't awesome. I love it and I am really convinced that it has revolutionized the way collaborative projects work. That doesn't mean it can't be better.
Being a distributed version control system, Git supports different workflows. Pull on demand is one of them, but mailing patches is an equally valid option. Git was initially conceived with the kernel's particular workflow in mind, and `git am` is fundamental in that process. Do I find it as easy to use as what GitHub offers? No, but supporting it wouldn't hurt and it would bring some old-school developers to GitHub's side.