| Phony Benoni: Perhaps not in this situation. Looking at this list of games with <5...c4>: http://www.chessgames.com/perl/ches...
You see a lot of Bronsteins and Korchnois on the Black side. This is an unusual line, but it's certainly worth taking a look to see what players of their caliber are thinking about. Compare the position that arises after <5...c4>: 
click for larger viewwith a similar position that might arise in the QGD Tarrasch after <1.d4 d5 2.c4 e6 3.Nc3 c5 4.cxd5 exd5 5.Nf3 c4>: 
click for larger viewIn both cases, Black has played ...c4 to gain space on the queenside and avoid the possible isolated pawn if the pawns on d4 and c5 are traded. In the French line, Black has no queenside majority, but the base of his pawn chain on d5 is more secure as it cannot be undermined by a pawn. The move can probably be played safely. Black even has ...b5 available, thanks to the knight on d2. The QGD version is almost never played. Even though Black has a queenside majority, his base on d5 can be undermined by White's e-pawn, which would leave the c4-pawn out on a limb. Nor is ...b5 immediately available, due to the knight on c3. Black would have to spend more time providing for the pawn's defense than getting on with life. There are many opportunities to push a pawn across the center of the board in the opening, gaining space but also exposing the pawn to danger. Not only immediate attacks, but undermining operations against the pawn's protectors must be taken into account. | 
Kibitzing
