The problem is rather hard. If Google manages to catch "only" 99.9% of spam links and does not penalize the receiving site for those, the 20 000 spam links ment

The problem is rather hard. If Google manages to catch "only" 99.9% of spam links and does not penalize the receiving site for those, the 20 000 spam links mentioned in the post would still add 20 links' worth of PageRank. As you point out, crappy links can be made to count for less; but creating spam links is really cheap, so they would need to count for very little to stop spammers. And stopping spammers is good - fighting web spam is (was) a major effort for e.g. forum operators.