As everyone knows Runners are commonly used for their amazing laying capacities. That would mean most people would probably prefer a female. Due to this having sex linked ducklings would be beneficial.
Hypothetically you had 1 drake and 4 hens. The Drake would be a chocolate runner while the ducks would be Black Runners. When breed the Drakelets would be black while the ducklets would be chocolate. Obviously these birds would not be beneficial for breeders because the next generation would not be sex linked and the genes would be of mixed colors. However, for a average backyard flock this would not be a big deal.