Class G and CTAF VMC below 3000ft AMSL/1000ft AGL is to facilitate VFR operations. The likelyhood of an IFR aircraft operating in this zone en-route is not great and close to CTAF & GAAP they should be communicating via the radio, and highly alert to the possibility of VFR traffic. CTAF with high volume of IFR traffic will be upgraded to CTAF(R) and the carriage of radio becomes mandatory.
Class C is set up to facilitate higher rates of IFR so VMC (and therefor VFR flight) is more restrictive. VMC may be reduced via special VFR to allow a VFR flight to arrive/depart a control zone so long as it does not delay an IFR flight.