Better to say it is your opinion that rod is the only suitable material for rudder shafts. It isn't a fact. For a given weight, tubes are stronger than rods. Many boat builders take this into account and use tube because it has other properties they want (strength/cost, easier to drill, etc). And then there are composite shafts that completely remove the corrosion and blade sealing issues. Most rudder issues are ones of corrosion or delamination. A skeg mostly delays or hides these problems. A skeg does help keep the rudder from being jammed against the stern upon impact, but that is really the only advantage (other than delaying a failure due to shaft weakness).

Mark