As Rusty Russell once said, if you want to get smarter, the thing to do is to hang out with smart people. An email-based workflow lets developers hang out with a project's smart people, making them all smarter. Greg wants Linux to last a long time, so wants to see the kernel project use tools that help to bring in new developers. Email, for all its flaws, is still better than anything else in that regard.
Good post on why email is ideal for kernel development process: