Line breeding of any animal is how we got so many breeds in such a (relatively) short time. How else did we get from wolf to dachsund or great dane in a matter of a few thousand years? But I think that many breeders take it too far just for the sake of looks, and that precious trophy: for example, many german shepards have bad hips due to too much inbreeding....... how does that make a better herding dog? It just LOOKS better, but in my opinion is no longer able to be a SHEEP HERDER.
Same goes for birds. We've gone from malards to Indian runners and Jumbo pekins. We have ducks that lay close to 350 eggs per year to jumbo Pekins that can weigh up to 12 or 13 pounds. Hardly any can fly, but we generally concider that a good trait (though the ducks might beg to differ).
Most of the differences we see beween the animals we raise and breed and their wild ancestors were caused by a mutation. The only way to ensure that that trait (the mutation) re-occurs and becomes its own strain is by line-breeding, otherwise the trait eventually gets lost. So those of you that are against line breeding should own only malards or (wild colored) muscovies, because while YOU may not be line-breeding, it is almost guaranteed that someone else in the past has.
If you're just trying to breed the same animals as copies of their parents, and you're happy with the way your ducks are, then you SHOULD breed from different genetic lines with similar features. It does genrally make for a more vigerous animal. Or just breed different breeds together for even more vigerous birds. However, if you're trying to improve on a certain trait then you almost HAVE TO line breed.