It proves nothing until you demonstrate that said person would have bought the books in question if there was no other option, otherwise it is meaningless.
There are all sorts of factors involved when you are dealing with content theft. You have to consider whether or not the 'lost sale' was actually lost first of all. This involves 2 factors. First would they have bought the product if they couldn't steal it? Second how many other products that they might never have bought did or will they now buy because they are now exposed to your product?
The second factor is meaningless to the point DracoSuave was making. Whether or not a pirate spent money elsewhere on product does not in any way impact the proof that the piracy of the CB lost revenue from the revenue stream of the CB. So we can put that factor completely aside.
DracoSuave made a very specific and provable point: Piracy of the CB takes away revenue from the potential revenue stream of the CB. And as he said... you'd need only a SINGLE person who pirates the CB instead of buying a subscription to DDI to prove that the revenue stream lost revenue.
Thus... the only way for this point NOT to be proven is if EVERY SINGLE PERSON WHO PIRATES THE CB WAS NOT GOING TO GET A SUBSCRIPTION TO GET IT OTHERWISE.
And I'm sorry... but if you honestly can sit here with a straight face and tell us that of the thousands upon thousands of people who pirated the CB that not a single one of them would have gotten a subscription to get it if they weren't able to just pirate it... you are just not being honest with us (or really, more to the point being honest with yourself) to try and make your point.
Look, it's okay... it is completely fine to admit that piracy DOES affect revenue for a company. The question then becomes HOW MUCH does it affect revenue... and for that, all we can do is speculate. But to try and say tell us it DOESN'T affect it AT ALL... even for a single $9... is completely untrue, and I think we all know this.