But the fact that other quality links can be liberally up-voted by others would prevent low quality links from rising. The only advantage I can see with the do

But the fact that other quality links can be liberally up-voted by others would prevent low quality links from rising. The only advantage I can see with the down-voting is that it will make (subjectively) quality links rise faster and poor links sink faster - but this is not necessarily an advantage, at least I don't think it is (I could be TOTALLY missing your point here!).