in the QGD Why Doesn't White just capture the d-pawn?
Because one of Black's problems is how to develop his white bishop when he blocks it in with his e pawn.
The trade takes that problem away.
Of course it has been played although it's not real popular. I copied this from a DVD advertisement on the web:
he Queen’s Gambit Declined Exchange Variation is one of the most important opening systems, having been played by most of the great players in history and from both sides of the board. The most outstanding specialists in this method of play include Garry Kasparov, Mikhail Botvinnik and Samuel Reshevsky whilst the Black side has been championed by such notables as Anatoly Karpov, Boris Spassky and Paul Keres. It is truly an opening of champions.
Characterised by an early resolution of central tension, the asymmetrical pawn structure leads to a variety of plans and ideas for both sides. One of the most traditional strategies for White is the minority attack which attempts to extend White’s influence along the half open c-file. There are also sharper plans available such as an advance of White’s central pawn majority or even castling long and going for a pawn storm on the kingside. On this DVD Nigel Davies explains these strategies in detail through a series of instructive games. Armed with this understanding the viewer will then be able to play this opening with either colour and always know what plans are available. Davies also shows how these plans can be used in openings with a similar pawn structure such as the Gruenfeld Defence and Caro-Kann, making this DVD valuable for those who would like to improve their knowledge of chess strategy in general.
In my experience when a pawn appears on d5, a large percentage of club players exchange even when there is no real reason to do so, i.e Bc8 can't be restricted.
I play the QGD as White a lot, and as a repertoire choice I use the Exchange Variation. Unlike other openings with pawn exchanges, White has some clear ideas exchanging here - and it is not just to simplify. First of all, this pawn structure favors White slightly after this exchange. Secondly, White is trading the c-pawn for Black's center e-pawn (central pawns are worth a little more). Also, (as some posts alluded to) Black can sometimes have trouble getting an advantage. When I play this line as White, I rarely attempt the minority attack though. White's position is a slower, but solid setup - and development with a little space advantage with the initiative are all common themes here. I think that the two best ways to play this as White (debate on best of course) is either:
1) the Nf3 lines with solid theory, played by many titled players today
or
2) planning f3 and e4 to challenge the center, in the style of Botvinnik
As Black, the QGD is always just conceding a small advantage to White - especially in the exchange structure. As Black, I would rather respond by a Slav/Semi-Slav compared to the QGD. However, many of these lines are sharp - and study is required; but it is one of the few ways that I think Black can fight for some advantage.
The exchange of the white's c-pawn for black's d-pawn often occurs, but just several moves later. The reason why is because white's DSB is often deployed to g5 to pin a knight on the f6 square, adding pressure to d5. By releasing this tension too early with cxd5, black has the option of playing a slav-like move of c6 rather than Nf6.
