Your example is not completely valid. The three major GOM operators fly IFR regularly. Literally tens of thousands of dual pilot, IFR operations a year from each of the larger bases along the coast. Icing is seldom an issue and the availability on fuel and offshore weather reporting stations make long routes common. Houston center can attest to their pace of activity.
When they return to their onshore base, they work within the same route structure and use the same approaches (ILS/RNAV/WAAS) as all the fixed wing "Hot Shots".
i can tell you from first hand experience, their IFR skills are not "lacking".