[zig]

In VMC you still have a responsibility to see and avoid, regardless of whether you're on an instrument flight plan or not. (Ref: Regulation 14 CFR Part 91.113 (b))

Radar coverage has become ubiquitous in most places, but there's not universal coverage. Heads-up time is very important unless you're flying in actual IMC.

[innocenat]

Can't find equivalent rule from ICAO, is this regulation US only?

[NovemberWhiskey]

Refer to ICAO Annex 11 - it's not the exact point being made by the GP, but note that traffic separation for IFR traffic from VFR traffic is only provided in Class A, B and C airspace.