Can anyone explain why Nikon put dual SD on a "pro" body? Many professionals prefer CF for many reasons. Even the upcoming 7d mk 2 has cf option.
I doubt Nikon will classify the D750 as a "pro" body.
The number one reason to put dual SD cards is that making the camera body smaller will appeal to enthusiasts and semi pro who travel and want a lighter camera. My guess is that Nikon sees a bigger market in enthusiasts and semi pros and photo journalists (not event / wedding pros) would may want a smaller body, priced between D610 and D810.
The advanced pro wanting CF slot with fastest memory cards available (faster than best SD) they probably think will just buy a D810. Of course, I am only guessing on their reasons, but would guess they would see a bigger market and better profit margin with this camera, and protect sales of the D810 for serious users and leave a reason to buy an expensive D810. If they gave it the D810 body, many photographers would just buy a D750, as 24MP fits the needs of many full time professionals (of course, not all pros), however, 24MP is enough for sports and journalists.
Wanting a D750 with a body/button configuration of D810 was perhaps too much to hope for.
Canon 5D3 got it right with enough MP for many professionals meeting their needs, and great choice for videographers and sports shooters. Canon 5D3 is still the number choice for serious pro videographers in North America if shooting with DSLR instead of bigger camera like Canon C series.
One can always shoot at lower resolution settings on a D810 if they want smaller files, however fullframe Raw is always very large files at 36MP unless using 1.2 crop or any crop the D810 allows.
The D810 or D750? Which to choose for next upgrade? At least there is a choice for a better than D610 now and price below D810 which is good for some.