Pete makes excellent points. Consider that sports photographers are the number one target customer and wildlife shooters a close number two. At working distances, for either, flash is of little relevance. While I don't mind having the option on my D700, I don't actually recall ever using it - even for fill - except testing that it actually does work. I spent a lifetime as a beast of burden, hauling a case of flash units and stands to please editors and clients when ISO400 was considered "high speed". I will die content to never having used a flash ever again.
I get medium-format quality at moderate ISO settings and even at ISO25,600, it rivals 35mm at ISO800 so long as you do not underexpose. Shooting in spotty light, I process my RAW exposures for each level and use layer masks to blend them together, rather than deal with the flat, harsh, ugly light of fill-flash. The key is to keep the histogram from hitting the right axis and blowing out the highlights. The D3/D700 sensor is very forgiving about shadow detail. If half the people on the street are in deep shadow, I can pull up all the detail needed, and still keep the feeling of naturally ambient lighting. Layers and masks give on ultimate control of the balance.