I never ever ever see this reply OTB or in live or online. It is a sharp and critical reply to the BDG "The Gunderdam". Book moves from the position at 6... c6 are 7. g4 and 7. Bc4 transposing in to a Zeigler....
OTB I would play 6. Bc4 then 7. 0-0 Black is up a pawn and white is developed but white has no "real" compensation for the pawn is something a GM might say... I say play it! The BDG is such a blast.
I would agree that the most likely time you would likely see this line is if someone specifically prepared it ahead of time before playing an OTB game. The idea with the way White is playing on moves 6 and 7 is to nudge the game back into familiar territory by making Black play the natural looking 7...Bg6.
I must agree with saxtusa. In my games with the BDG, I never ever see this move.
In my database there's 13500 games in the Blackmar-Diemer Gambit and this variation has only ocurred once
I have a pretty good understanding of the different lines in the BDG, but the line below (to me) feels like it busts the opening. Black enjoys an advantage and there aren't any traps for White. I'm open to counter-arguments...