I think the essential point is the reason for instrument flying in the first place, i.e. navigation without reference to the ground, which means stricter procedures to make it safer.
When VFR, you don't *have* to keep minimum distances necessarily, unless in airspace that requires it or your Ops Manual says otherwise.
If it helps any, the UK ANO IFR conditions cover minimum height, flight plans and reporting for controlled airspace, and minimum height and quadrantals for uncontrolled airspace.
Phil
[This message has been edited by paco (edited 01 May 2001).]