The primary argument I've heard for the leaked rule book being real follows:
A real 40K rule book takea a lot of effort to write.
This leaked rule book took a lot of effort to write.
Therefor This leaked rule book is a real rule book.
All A is a B
C is a B
Therefore C is an A
To make it clear, lets Venn it out
Just because something takes effort does not mean that it is real, only that it could be. Yet, this is a common argument in support of it,
Another fallacy I see in relation to the leak is wishful thinking. Since the leak matches what people want the next edition to be like, they are more inclined to believe it.
Then there's this gem from a guy by the name of Stucarius:
Since there is no evidence that is is not true, it must be true. That does not logically follow. This is what is known as an argument from ignorance(not trying to be mean, that's really the name of the fallacy). Absence of evidence is not evidence of absence.“The work that has gone into this set of rules is legion. There are not that many designers in this community who could do it and I cannot imagine anyone who would and then not take credit.”To be honest there is absolutely no evidence I have seen so far that points to this not being a leaked copy or a near final draft of the rules. Very much the opposite actually. All the circumstantial and logical evidence, not to mention history (see the leak of 5th ed) point to this being the rules.
There's also a false dilemma. It presents the choice that its real or its being done by someone who would intentionally not take credit for it. This is denying the possibility of other reasonable assumptions, like the author is unavailable to take credit or doesn't want to be sued by GW. The whole argument right now is being presented as a false dilemma. Either it's real or a hoax. It's denying the very real possibility that it is someones pet project; not something done to intentionally deceive.
There's some correlation is not causation action going on too. Just because the 5th edition leak turned out to be real, does not mean that the 6th edition leak will turn out to be real.
The leak could very well be true. Tearing apart the arguments, does not change the truth of the matter. But if the arguments for the potion are bad, and the evidence for the position is bad, it is most likely that the position should not be affirmed. The evidence against the hoax is just about as bad as the evidence for. To me that's a push. I'll wait and see what happens.
I do think it's interesting that with equaly poor evidence for and against most people seam to believe the leak is real. I think that's because most people want to believe what they are reading is really 6th edition. Unfortunately wanting something cannot change reality.