I find it puzzling that atheism is viewed as such an important part or maybe the most important of a person's identity. Well, it's only puzzling if what I repeatedly hear from atheists that it's a simple non belief. I, personally, give very little thought to things I don't believe in. I don't believe in flying ponies. Let's have a non belief in flying pony rally.
On the other hand, my own religion is a very large part of my identity and that I would rally for, or I might rally against something I'm hostile toward. Is this an anti-religion rally? That doesn't seem like that's what Penn wants. I think an event like this is all good but I also think atheists who truly don't see their atheism as any kind of big deal would be unlikely the ones to go to this, so that I think the hostile, mocking, evangelical atheists will be over represented and Penn's idea that this will showcase that atheists are nice, caring, and "normal" probably won't pan out, but we'll see.