Hi Andreas and welcome to the forum,

well, when creating shapes in MoI and bringing them into ZBrush there is no need to further subdivide,
imho, unless of course you would create a quad only low-res mesh in MoI. Usually one brings CAD
models into ZBrush for retopo and then further processing.

Regarding your second question, the subd2nurbs feature in MoI is very useful if you like to create
organic shapes, which are usually difficult to model with CAD Software. Once those organic shapes
are converted you can then treat them as NURBS shapes for further processing, like booleans,
trimming, detailing etc.

Hope this helps!

Regards
Stefan