Snape's textbook and the purpose of Slughorn being Potions professor

[Bekki]

No: HPFGUIDX 133917

>Juli now: Dumbledore "needs" Slughorn back because he needs the 
> conversation he had with Tom Riddle about Horcruxes, it's crucial to 
> the whole battle against Voldemort.


Bekki:

But why was Slughorn a Potions teacher instead of a DADA teacher? Maybe:

1) Slughorn's frequent comments about Lily's potions abilities are somehow significant/
revealing.

2) Slughorn needs to be at Hogwarts in the next book. However, Harry doesn't plan to 
return to Hogwarts.

3) Harry needs a Potions NEWT to become an auror, and Snape wouldn't let anyone without 
an O OWL in the class. However, J.K. Rowling made up this restriction herself! 

4) I've heard people say it's proof that Dumbledore knows Snape will only be here this 
year: there is a plan for him to go "deep uncover."

5) A horcrux sounds more like something a DADA professor would know about than a 
Potions professor unless, as I've read other people suggest, the creation of a Horcrux is 
connected to a Potion.

6) As Ersatz Harry said, it's a way for Harry to get the Half Blood Prince's book. Howevever, 
I wonder how would this have been managed if the HBP strand was part of CoS.

Or is there some other reason?
Bekki