Why would you use a DX lens on a FX camera
I realise the D3x has lots of pixels so it's DX mode will still have decent resolution but it seems a little odd...
The D3x has a 10million+ pixel crop mode which is fine for prints that will only be printed to 10x8.
I needed a zoom to get a variety of crops from one shooting point for a shool portrait job. The distance from me to the subject, and then the subject to the background, in the space I was given to shoot, meant that I needed a 40 -120 mm focal range. I have had 4 copies of the 24-120 mk1, and they were all soft. The 24-120 f4 wasn't available at the time, and kit bag included 8mm, 14-24mm, 50mm, 105mm, 80-200mm, 200mm, 500mm
There where 2 options.
1 -buy a used 24-70 at over £1k that wouldn't get used much, and use the camera in crop mode.
2- buy a used 16-85 to use in crop mode for a third of the price. There were also a couple of jobs each year where I didn't like taking expensive glass, so the 16-85 was better suited.
The 16-85 is an excellent little lens for crop cameras, the 24-120 f4 was obviously better suited, but wasn't available when I needed that particular range of focal length. I sold the 16-85 when the 24-120 f4 came out.
It is all good having the "standard" focal ranges - something to 24, 24-70, 70-200, but in the real world the there are often times where your subject falls in the 18-50, 50-100, 100-300 range, and the manufacturers don't really cater for that at pro level - ie, full frame / @2.8 or wider.