I'm just catching up here.
The reason the debates about atheism sound a bit "tired" and "hackneyed" is that there are only so many ways you can keep on saying the same thing. We try our best to offer sparklingly rhetorical ripostes, but even I run out after a while.
It's like that new insult, "militant atheist". That would be one who persists in their awkward and irritating way in not believing in god despite having been told again and again that they are wrong, wouldn't it?...
I think the "imaginary friend" thing is, mmm, okay, maybe on the rude side, but it comes out of frustration and exasperation. It's another rhetorical device, if you like, a way of trying desperately - trying any way - to get through to cloth-eared theists (not that they are all cloth-eared, but I meet a lot who are) who Just Don't Get What Atheist Means. It's come after decades of the debating equivalent of banging one's head against a brick wall.
It's a way of trying to say "look, what if your god was just an imaginary friend? And you believed it to be real? How would that situation manifest itself? And how would that be different from the one we are talking about now?" I think thats quite interesting, philosophically.
And yes, no proof. No proof given, no proof demanded. Because you can't prove it either way. But if you can't prove it either way, there has to be something else to justify it, otherwise any old crap - fairies, imaginary friends, dragons - is eligible to be let through the portal of belief. There has to be "gatekeeping", and that gatekeeping is rationalism.
The person making the extraordinary claim has to offer the evidence. Always. It's just that, to a lot of people who have a religious belief, it doesn't seem that extraordinary, or in need of justifying with evidence. And I understand that. Because they are inside it, and cannot see beyond it, and it will always be so.