Dead Horse, we mostly agree, but I think the repeated solo lines were perfectly normal and typical. Hugill says he heard several old but not forgetful chanteymen sing that way, and the vast majority of chanteys collected by Sharp, Grainger, Carpenter, and others from actual performance repeat the solo line.

The advantage, as you say, would be to give the singer more time to ad lib if necessary. But that's not to say that only poor chanteymen sang that way. The evidence is otherwise.

If they'd been singing in folk clubs, they'd undoubtedly have changed their habits. But they weren't.