Originally posted by Golub
I never understood those "play 1..Nc6 against everything" kind of books. It's usually two completely different systems anyway (even with similiar structures). It's not like you really narrow down anything (if you think that is good).
As for the actual topic, I think there is a book on the a6-slav, but I do not remember the name (a system I think is kind of interesting). I think it is supposed to be good.
I basically agree with your points. However, pawn structure often dictates middlegame plans, and if almost all of your games involve a pawn on c6, that narrows the number of middlegame plans that need to be mastered.
Also, if you play the Slav vs. 1 d4 and the Sicilian vs. 1 e4, what do you do after 1 d4 d5 2 e4? If you also play a Caro-Kann, then you just play 2...c6 and you're in familiar territory. Sometimes 1 Nf3 and 1 c4 openings transpose into 1 d4 systems, so dealing with transpositons if less tricky if you have a familiar ...c6 and ...d5 plan.