This would be valuable in many cases as witnessed by users interest in the following threads:
EDIT: forgot link to issue https://freecadweb.org/tracker/view.php?id=3466
AFAIC, in a woodworker/cabinetmaker use case, this feature would make my base parametric parts, cabinets/furnitures, assemblies much more powerful:
* cabinets with/without shelves/separations,...
* panels with/without edge banding, face veneering
For BoM/cutlist generation, an "Enabled" boolean property (like proposed in second linked thread, even a "dumb" one) would be further empowering by allowing to manage visibility and "existence" independently.
In the meantime, I'll take all ideas to create "optional" parts