Because he knows the theory.

Why isn't the Diemer-Duhm Gambit, 1 d4 d5 2 e4 e6 3 c4, discussed here? All I am hearing about is the Alapin-Diemer Gambit!

Let me start off by stating that I have books that deal with this gambit 

1) Lane - "Four Gambits to beat the French" 

2) Tim Sawyer - "Alapin French Tactics for White"

I have played this gambit in both correspondence and over the board chess.

First I strongly believe that black should accept this gambit.

Declining the gambit tends to give black inferior positions.

For example 

1. d4 d5 2.e4 e6 (the move order that will often happen to Blackmar-Diemer gambit players.
3.Be3 Nf6 4.e5 Nd7 5.c3 and a position is reached where white has supported his centre with c3 and Be3 before black can put pressure on it. Black will have more trouble drumming up counterplay than in other lines of the French.

On the other hand in my opinion black is better after taking the pawn. 

From my head, isnt Qe1,Qh4 and if h6,something like Bh6 or even Nf7 supposed to be really dangerous for black?


f.e. 

8 Qe1 ! c5
9 Qh4 and if h6
10 Bh6 gh6
11 Qh6 really dangerous,white plays Ng5 next move ?

I play it safe with 3..Nf6  

Just earlier today I in the national league I deviated from my normal Sicilian and faced:

1 e4 e6 2 d4 d5 3 Be3  So I figure I should contribute

I didn't know anything about it but since I always take the Morra and BDG pawn I decided that it couldn't be much worse.  The game continued:

3...dxe4 4 f3 exf3 5 Nxf3 Nf6 6 Bd3 Be7 7 O-O O-O

reaching a position where if Nc3 is swapped with Be3 it is the BDG, which continues Bg5 etc.  In any case, I was here going to play 8...c5 almost regardless of what white played

8 Ne5 c5 9 c3 (9 dxc5 Qc7/Qd5) 9...cxd4 10 cxd4 Nc6! 11 Kh1!? (11 Nxc6 bxc6 is a c3 sicilian type position where white is missing his f2 pawn.  11 Ng4 Nxg4 12 Qxg4 f5 would have been nice) 11...Nxe5 12 dxe5 Nd7 (12...Nd5!? 13 Bd4 Nb4 14 Be4 Nc6 forcing more exchanges was simpler and much better for black, but I had missed a trick) 13 Bd4 (13 Bf4 Nc5)

I had been aiming for this position and here made a mistake with 13...Bc5?! (13... Nb8-c6 is the computers nice suggestion) missing that while 14 Bxh7+ Kxh7 15 Qh5+ Kg8 16 Rf4 Bxd4 17 Rh4 f6! is winning for black, white can instead play:

14 Bxh7+ Kxh7 15 Qc2+! Kg8 16 Bxc5 and here 16...Qc7 17 Rc1 Nxc5 18 Qxc5 Qxc5 19 Rxc5 Bd7 is only a little better for black but not a disaster for white.

Fortunately (for me) my opponent instead played 14 Bxh7+ Kxh7 15 Bxc5?? Nxc5 16 Qc2+ Qd3 0-1

So while I did indeed fall into a "trap" (that gave me slightly nicer ending) despite knowing nothing about the theory I reached a comfortably better position (and my opponent fell into a bigger trap).  I can't say I will spend any more time studying the line as a result. (my opponent was rated around 2000)

I think your evaluation of the position after 3...Nf6 is somewhat exaggerated, but true, White has an extra tempo on the interesting Tromp line 1. d4 Nf6 2. Bg5 Ne4 3. Bf4 d5 4. f3 Nf6 5. e4 e6 6. e5 Nfd7 7. Be3, which must be worth something.

Neither have I. From the 100 times I meet the French I will play about 99 times 3.Nc3.