The ONLY time inadvertant IMC may occur is at night. Simple as that.
I disagree. Anyone who flies at night and does not have a plan for penetrating clouds or areas of darkness with no visible horizon is a complete menace IMHO.
Undertaking night flying by visual reference is also a choice and there must be a valid contingency plan before making that choice.
If you flew me through a rain shower that lost us the required VMC minima on flight test I would automatically fail you.
Also I should point out that it is in-flight visibility (i.e. directly in front of you) NOT your ability to see the ground 3km away - the point of the VMC minima is to avoid collisions amongst other things...
So tell me, how do you definitively measure in flight viz directly in front - i.e. without any reference?