I would like to remind us all that every time you change a lens, you allow dust to come in to your camera. In addition, every time you hold the lens by one hand and the camera by the other, there is a small chance of dropping one of them (or both).
I try to minimize the number of lens changes. Especially with my D3 which doesn't have the dust removal. But I'm sure that is not an option for the professionals.
Of course, there is one more option than having zooms or switching fixed (or any) lenses: buy a couple more camera bodies! It seems that everyone has to do a compromise of one sort or another.
I do agree that a lens like 50 mm f/1.x is very good in low light performance and very light to carry but you are also stuck to just one cropping mode and if you happen to get too close to your subject, there's nothing you can do. However, even if you can frame the picture perfectly using a fixed lens, there is the DOF issue (or advantage, of course) which may force you to use a smaller aperture anyway.
I'll stay with the 24-70 in D800 and 70-200 in D3.