I use my 50mm as an intermediate between the 14-24mm 2.8 and 70-200 2.8 in my carry around bag. Those lenses are pretty big, it's nice sometimes to have a smaller lens on. Also, at 1.4, I like the bokeh effect. It's handy for low light, although on the D700 with the camera set to min. 1/30 shutter and control exposure with ISO, I rarely worry about that TBH. I just walk back or forward a bit, when the situation allows it, otherwise one of the other lenses. I guess it just connects the range a bit.
Edit: I see you are using a DX sensor, but I think the principle still applies.