Quote:

I have to agree with that statement. I never come into trouble against it and the play is very easy and natural for black. The only problems OTB I have encountered were in the variations with 4 Nf3 or 4 Qg4, but mainly because I had never bothered to look them up.

The most annoying variation I have encoutered is the exchange with an early c4. The play is not quite along the standard french lines and very similar to the Panov attack in the CK. From a white perspective it cuts out a lot of theory, without having given black a clear plan.

Well, I think that actually a3 is horrible against Bd7 since that's the best closed variation black can normally get and has some great setup options, as for the reasons Be2 isn't best against Qb6 is that after cd cd Nh6 it is already becoming annoying to defend d4 properly, the main line after Nc3 Nf5 Na4 has a problem after Qa5+ Bd2 Bb4 and black has achieved at least full equality.  However compare this to the lines with a3 (intending b4 if black doesn't play c4) where white has a nice way to defend d4 by Bb2 and white often gets a solid position.  However the whole advanced french may be the most unnatural system against the french even though the third move e5 seems natural because white's attacking plans are hardest to find and it often is hard press without taking some serious risks.

The reason I think is because in the a3 advance black often plays c4 (to annoy b4, which if just allowed is a pain for black). Once that is done black would want to block up with moves like Ba4, Na5. To get the bishop to a4 a knight on b6 is very helpfull. With the Bd7 move order b6 is not aken from the knights and the bishop needs one move less to get there. With the Qb6 move order black still needs to play Bd7 and remove the queen to execute this plan. Still playing c4 in that variation is not bad at all, though for some reason people now play Nh6 iso c4

The disadvantage of Bd7 is that it doesnt put any pressure on the centre, so that white can simply play a good developing move. Because after a usual cxd4, cxd4 black would wish he could replace his Bd7 move with Qb6. Note that it is all very much nuances as the position is quite closed and even if either side plays a few inaccurate moves he will hardly be lost. 

One word of warning with the advance variation. It is very easy for white to play without purpose and get creamed in an endgame, without having made any obvious errors. At least that is what happens a lot in my games.

But after 1.e4 e6 2.d4 d5 3.e5 c5 4.c3 Nc6 5.Nf3 Bd7 6.Be2 Black can still play 6...Qb6 if he wants to e.g. 7. O-O cxd4 8. cxd4 Nge7 9. Na3 Nf5 10. Nc2 h5 etc.

It is actually quite simple.

The black queen is simply not ideally placed on b6 in the a3-lines, so black often has to spend a whole tempo just on moving it away. Therefore a3 is best against 5...Qb6. Whether white is able to get an advantage though is another issue...

With the queen still on d8, the a3-lines are improved for black, thus 6.Be2 is usually prefered, although 6.a3 is still possible.