Queens Gambit Declined with 1.Nf6

That discussion in Sadler is in the introduction, at questions 14-17. The point of the move order with 1...Nf6 and 2...e6 is that Black makes it appear as if he intends to play the Nimzo. Many players don't want to play the White side of the Nimzo, so they will play 3. Nf3. Then Black can play 3...d5 and have his QGD without worrying about Exchange Variation lines in which White's KN goes to e2.

If White welcomes the possibility of a Nimzo by playing 3. Nc3, you're right: Black can continue 3...d5, in which case his move order was a bluff. The game continues as a regular QGD. But Black has at least kept White guessing and given him a chance to commit his N to f3.

With 1...Nf6 one could also adept a more flexible setup against London and other queen's pawn games. Not unimportant nowdays.

Good point. I even go one step further by playing 1...e6 against 1. d4. If White wants a French Defense, that's great. If he plays 2. c4, my usual move is 2...Nf6, going into the lines brought up by the OP. I play against the London a couple of different ways, depending on my mood.

Hmm, interesting. Same question I have been pondering for a few weeks or so... playing a QGD or classic formation, but opening with the knight, make the opponent wonder what intention you having? A sort of Fake Indian defence. Doing such with the Slav is named Slav Indian defence, but I don't know what sort of reputation that has? Only negative is crap like the trompowsky or levitsky attack, but those should be fairly easy to deal with. 

That's obviously not true or all GMs would be playing this way. There is a reason for why they don't start with Nf6, there are obviously other negatives.  

Such as?

Most GM play the QGD as a drawing tactic, yes? What about the Slav Indian?

If I wanted to expand on it I would have already. I'm stating that by clear logic there is a move order reason why grandmasters play it that doesn't involve "crap like the trompowsky or levitsky attack". 

Indeed. I only really play X. ... Nf6 if white plays X. Nf3. It is true some white players don't want to play certain Indian-type defenses but for me I really dislike locking in my LSB soo early in the opening playing X. ... e6 unless we're both all-in on a QG-type game. Playing 1. Nf3 Nf6 signals to both sides it's probably going to be a English, KIA, Catalan kind of theme. Cool.

@Spaghetti troll... you can't say more because you don't have any more. Clear logic, but you don't know what the logic be... And for your information, I checked... playing knight f6 to get a QGD or Slav is Very common and has been used by WC players. And is the Most popular reply. So I think we listen to them... rather than a patzer like you? Dismissed.

That's an annoying bluff for sure.