If you photograph people, the focus assist light can be piercing in their eyes and extremely annoying. It also slows you down as it increases the time lapse between your pressing the shutter release button and the actual taking of the photograph. I frankly haven't been in any situation in which the camera actually required the AF assist, so I disabled it almost right away, together with the beep alert.
>So, do I understand correctly, that the AF assist light is >being activated only of I use the internal flash?
The AF-assist lamp is there because the D700 has a built-in flash, to allow shooting in dark conditions without needing to add a Speedlight. But (as long as you have not disabled the function), the AF-assist lamp will illuminate if it's too dark to focus whether you have the built-in flash popped up or not.
For what it's worth, the on board AF assist is visible light. In a word, it sucks. The AF assist on speedlights is IR and that is very nice. Use the AF assist on Speedlights but disable it on the body.
I always had it turned on, also on my D100 and D200. Yesterday i bought the 24-70mm 2.8 and i noticed the light is not right on the focusing spot any more. The focusing spot is in the shadow of the big hood.